Abstract: The drilling string in extra-deep wells is subjected to strong downhole impact torque during drilling, and its safety is facing severe challenges. Therefore, it is important to measure and analyze the downhole impact torque of drilling strings in extra-deep wells. First, the three-dimensional (3D) mechanical analysis method was used to obtain the mechanical characteristics of drill collar joints under different torque. Using that information, the corresponding relationship between the relative offset between internal and external threads and the downhole equivalent impact torque was established. Then, with the NC56 joint of ϕ203.2 mm drill collar in the upper section of an extra-deep well in an oilfield of western China as an example, the relative offset between internal and external thread shoulders was measured by the scratching mark, and the downhole equivalent impact torque corresponding to the offset was obtained through inversion and verified with the measured breakout torque of the drill collar joint. The measured data show that there was a large downhole impact torque during drilling, which can cause the drill collar joint to bring about downhole secondary makeup. The downhole equivalent impact torque calculated by the inversion of the measured offset is well consistent with the breakout torque, which shows a good reliability of the method. The results indicate that the pre-tightening state exerts 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 secondary makeup torque.