Star sensor is a high-precision space attitude measurement instrument with high precision, good autonomy and independent existence of other systems. It takes the starry sky as the working background and stars as the benchmark to obtain the attitude information of the spacecraft by detecting stars in different positions in space. Therefore, its accuracy is the key factor affecting the overall performance of the whole system. The all day star sensor is a star sensor that can still detect stars under the strong background in the daytime and has the anti-interference ability to the strong sky background. As the most important part of the optical system, its imaging quality is very important to improve the star detection ability of the star sensor. However, with the development of aerospace technology, space science has higher and higher requirements for the attitude accuracy of spacecraft. Therefore, in order to meet the needs of all-time high-precision detection, the lens of the star sensor optical system must adopt a large relative aperture to improve the star detection ability. In order to realize the all-time high-precision detection of class 3 stars by star sensor in J-band, this paper adopts the method of passive thermal difference design, carries out matching optimization according to the thermal difference performance difference between the optical system and structural materials, and then realizes lens thermal difference elimination. An all-time star sensor optical system with a large relative aperture is designed and completed. Firstly, the irradiance and signal-to-noise ratio of class 3 stars in the J-band are analyzed to determine the main parameters of the optical system, in which the focal length is 84 mm, the F number is 1.4, and the working spectrum range is 1.1 ~ 1.4 μm. The field angle is 8.4°. Secondly, considering that the optical system of the star sensor has the characteristics of a large relative aperture, long focal length and the influence of optical system distortion on the accuracy of star point extraction, the distortion free telephoto objective is selected as the initial structure of the optical system for optimization. In the process of optical system design, common optical materials and lens barrel materials are selected. By changing the shape and thickness of each lens, the focal power and air gap between each lens are reasonably matched, so as to realize the passive compensation non-heating design. After the optimized design, the dispersion spot size of the optical system is better than 30 when the defocus is 0.02 mm under the conditions of high and low temperature (-40 ℃~ +60 ℃) and vacuum μm. The color distortion is less than 0.018 mm, and the design results meet the design requirements. The inner surface of the star sensor is blackened, the light shield is designed with non-equidistant layout, and the surface is blackened with an SB-3A domestic extinction paint with high solar absorption, which can effectively reduce the weight under the condition of ensuring the effect. The inner baffle ring of the light shield adopts a 16 ° oblique angle, which can ensure good stray light suppression ability. The stray light of the optical mechanical system is simulated and analyzed by using TracePro software. The analysis results show that the stray light generated by the target in the field of view is 3×10^-5 of the intensity of the target, the stray light intensity outside the field of view decreases rapidly from the order of 10^-2, and the stray light intensity outside 18° is less than 10^-4 of the strong light outside the field of view. Finally, the actual ground star observation test is carried out on the principle prototype. Through the star photos and three-dimensional energy diagram taken by the principle prototype, it can be seen intuitively that the signal intensity of the class 3 star target is much greater than the background intensity. After subsequent image processing, a clearer star observation effect can be obtained. Through theoretical analysis and design and practical observation experiments, it is verified that the optical system designed in this paper can meet the requirements of all-time high-precision detection of class 3 stars in J-band, which also shows the rationality of the design of the optical system.