To solve the problems of the small spot of graded-index lenses and the significant coupling loss and short effective alignment time of the current direct alignment-type off-axis fiber optic rotary joint， this study devised a large-spot fiber collimator made of a double-lens collimator and thermal expansion core fiber. Based on the periodic variation law of the overlapping area when the exit spot rotates relatively， the change in the coupling loss when there are radial and angular errors between the collimator was analyzed. The theoretical effective alignment time of the off-axis fiber optic rotary joint under a 3-dB coupling loss was calculated from the exit spot size of the rotor ring and the fiber collimator. An optical fiber coupling experimental platform was set up to test the coupling performance of the off-axis fiber optic rotary joint and measure the effective alignment time. The experimental results indicate that a larger mode field diameter （MFD） of the single-mode fiber used in the fiber collimator corresponds to a smaller coupling loss. In the test， a large MFD of 28 µm yields the minimum loss of 0.22 dB. At a speed of 60 r/min， the effective alignment times of the four MFD fiber collimators were obtained via photoelectric conversion and function fitting. For the MFD of 17 μm， the maximum effective alignment time is 7.05 ms， which is close to the theoretical value of 7.1672 ms. Finally， fiber Bragg grating sensing was applied to achieve contactless transmission of optical signals. This design provides theoretical and technical support for the large-spot off-axis fiber optic rotary joint and its application in optical fiber sensing.