Abstract: To address the challenges of accurately evaluating the instantaneous impact force and unblocking effectiveness of downhole drilling jars during impact unblocking operations in deep and ultra-deep wells, this paper takes a 121 mm drilling jar as an example and designs and develops an impact force acquisition sub suitable for the high-temperature and high-pressure environment of ultra-deep wells. The tool has an overall length of 1100 mm, an outer diameter of 127 mm, a rated operating temperature of 150 °C, and a compressive strength of 75 MPa. It employs resistance strain gauge technology as the measurement method. Its functional components include resistance strain gauges, an integrated circuit board, a battery pack, a damping device, a temperature sensor, and a pressure sensor. During operation, the tool can automatically switch between high- and low-frequency acquisition modes through the parallel activation of pressure and acceleration sensors, thereby meeting the requirements for impact force acquisition over long periods in downhole environments. Finite element strength verification, simulated well tests, and surface bench tests have confirmed that the tool exhibits reliable strength, good sealing integrity, and stable acquisition performance under high-temperature and high-pressure conditions. It can accurately capture the axial load fluctuations in the wellbore drill string during the jarring-and-freeing process, providing a direct and reliable measurement means for the quantitative evaluation of the jarring performance of downhole drilling jars.