Abstract: The drill string in eltra-deep wells is subjected to strong downhole impact torque, and its safety is facing severe challenges. Therefore, it is important to measure and analyze the impact torque of drilling strings in extra-deep wells. Three-dimensional elastic-plastic finite element analysis method is used to obtain the mechanical characteristics of drill collar joint under different torque, and the corresponding relationship between the relative offset between internal and external threads and the downhole equivalent impact torque is established. Then, taking the NC56 tool joint of ϕ203.2 mm drill collar in the upper section of an extra-deep well in western oilfield as an example, the relative offset is measured by the notch method, and the downhole equivalent impact torque corresponding to the offset is obtained and verified with the measured breakout torque of drill collar joint. The actual measured data show that there exists a large equivalent impact torque in the drill string during drilling, which can cause the drill collar joint to make downhole secondary makeup. The downhole equivalent impact torque calculated by the inversion of the measured offset is in good agreement with the breakout torque, which shows that the method has good reliability. The results indicate that the pre-tightening state has a great influence on the downhole secondary makeup characteristics of the drill collar joint, and avoiding under-torque makeup is an effective means to prevent the drill collar joint from being secondary makeup.