Flexure shaft is a key component for torque transfer between the joints of ultra-short radius flexible motor units. Aiming at the failure phenomena such as fracture of flexure shaft in the drilling process of ultra-short radius flexible motor, this paper carries out mechanical modelling and design strength checking of the internal flexure shaft of the flexible motor, uses finite element simulation to carry out stress analysis on the stress state of the flexure shaft in the torque transmission process of the flexible motor, and tests the mechanical properties of the flexible motor through indoor tests to analyse the mechanical properties of flexure shafts. The results show that the root and spline contact surfaces of the flexure shaft are the stress concentration areas in torque transmission, and the fatigue life of the flexure shaft can meet the requirements of continuous downhole operation. The finite element analysis and indoor tests show that the flexure shaft can transmit torque of 500N·m, which can realise the internal torque load transmission of the flexible motor under alternating bending and torsional stresses. This provides an important theoretical basis for the drilling process of ultra-short radius flexible motors in the development of horizontal wells, and provides a new type of technical means to improve the drilling length and drilling efficiency of the horizontal section of ultra-short radius horizontal wells.