A loading system for performance test of servo systems in robots is designed by using a programmable logic controller S7-1500 and a variable frequency converter Sinamics S120. The compositions of performance test system, hardware and software designs of the loading system are presented. The close-loop control of loading system was modeled, and the torque proportional-integral controller and torque differential negative feedback controller are introduced to eliminate static torque errors and suppress torque overshoots. The practical application results show that the system performance indicators can meet design requirements.