As an important part of modern communication system, satellite communication undertakes important tasks such as communication, earth observation, navigation and positioning in military and civil fields. The traditional spaceborne optoelectronic load realizes the data signal and power transmission between the relative rotating bodies through the slip rings. With the continuous development of optical fiber technology and related components, laser communication with optical fiber as the transmission medium has gradually replaced the traditional signal transmission with wires. The fiber optic rotary joints have the characteristics of a wide communication frequency band, strong anti-electromagnetic interference ability, strong confidentiality ability, fast transmission rate, low loss, etc. Its performance largely determines the service life of the satellite. Low loss and high reliability are important indicators of single-channel fiber optic rotary joints. This paper takes the single-channel fiber optic rotary joints as the research object. In order to achieve its low loss and high reliability goals, it is necessary to explore the factors affecting the insertion loss. The gap between the single-mode fiber and the gradient-index lens and the position error between the two gradient-index lens collimators are all important factors that affect the insertion loss of the fiber optic rotary connector. The Gaussian beam coupling has attracted the attention of universities and research institutions from all over the world. But the previous analysis ignored the influence of the position error between the fiber and the gradient-index lens on the coupling efficiency. There is no corresponding compensation method for the above-mentioned errors, which is crucial for improving performance parameters and reducing the difficulty of processing and assembly. This paper takes the single-channel fiber optic rotary joints as the research object. In order to achieve the goals of low loss and high reliability, it is necessary to explore the factors affecting the insertion loss. The fiber optic rotary connector studied in this paper uses two gradient-index lens collimators as the main optics. Theoretically, the propagation model of Gaussian beam in the construction of gradient-index lens is established, and the optical characterization parameters of the gradient-index lens are obtained by mathematical analysis method of light transmission matrix. In order to describe the propagation of the Gaussian beam in the gradient-index lens, the (x, y, z) and (x', y', z') coordinate systems are established, and the electric field vector equations are established for the lenses at the receiving end and the transmitting end. Based on this equation, the influence of lateral offsets on the coupling efficiency of the system is discussed. Using the geometrical optics analysis method, the energy distribution equation under the separation misalignment is established, and the influence of the separation misalignment on the coupling efficiency of the system is analyzed.This paper design the single-channel fiber optic rotary joints with low loss as the key parameter by ZEMAX, and the optical model of the single-channel fiber optic rotary joints is established, and the optical parameters of the gradient-index lens are preliminarily determined. For the convenience of processing and assembly, the two gradient-index lenses are designed with the same parameters. First, without changing the working distance, set the distances to 0, 0.05 mm, 0.10 mm, 0.15 mm, 0.20 mm, and 0.25 mm between the optical fiber at the transmitting end and the gradient-index lens. In order to obtain the insertion loss at different positions, the value of the fiber at the receiving end and the gradient-index lens is changed. It can be seen from the analysis that the same insertion loss as the initial value can be obtained by adjusting the position of the optical fiber. This method can reduce the influence of the error between the optical fiber and the gradient-index lens.Secondly, by changing the lateral offsets and separation misalignment of the two gradient-index lenses, the effects of lateral offsets and separation misalignment on the insertion loss of the system are obtained. It should be noted that due to the particularity of the gradient-index lens, the lateral offsets cannot be so large that the Gaussian beam cannot be coupled into the fiber. The axial distance is controlled within 0~14 mm, and the radial distance is controlled within 0~0.25 mm. It can be seen from the simulation that the lateral offsets have a great influence on the insertion loss of the system, and it is necessary to strictly ensure the accuracy in processing and assembly.In view of the above errors, the insertion loss is reduced to 0.2 dB by the displacement method, which provides a reference for the optimal design of the single-channel fiber optic rotary joints. For the separation misalignment and lateral offsets between two gradient-index lenses, a beam steering technology based on wedge prism and flat glass is proposed. This method mainly uses two wedge prisms to achieve beam steering, the flat glass adjusts the transmission optical axis and the receiving optical axis to be on the same axis as possible. The insertion loss of systems can be reduced to 0.7 dB by beam steering technology, which greatly reduces the influence of errors. The difficulty of processing and assembly is reduced, and the reliability of the system can be improved.Finally, a test system for the insertion loss of a single-channel fiber optic rotary joints was built, and the position of the optical fiber and the gradient-index lens was adjusted with a high precision fiber alignment stage, and observed through a binocular microscope. By fitting the experimental data with the simulation data, the accuracy of the system design and simulation analysis is verified.