Abstract: The common characteristics of threaded connection structures in inertial devices are small size and high tightening quality requirements. In this paper, a set of tightening test devices for small-size screws was developed to address the problems of large preload error and low control accuracy in the assembly process of small-size screws. The effect of the threshold torque and stepwise tightening on the preload was further investigated. The results show that, compared with the torque method, the torque angle method can improve the accuracy of controlling the preload by about 7%; the selection of the threshold torque when using the torque angle method has a greater influence on the consistency of the preload; stepwise tightening can improve the accuracy of controlling the preload force by the torque method and the torque angle method.