The invention discloses a motor performance testing system which comprises a driving device, a motor to be tested, a loading control device, loading equipment, and data acquisition and analysis equipment. The driving device is electrically connected to the motor to be tested. The output shaft of the motor to be tested is axially connected to one end of a transmission shaft through a coupler. The other end of the transmission shaft is connected to the loading equipment through a transmission gear set. The loading equipment is electrically connected to the loading control device. The coupler is equipped with a torque rotation speed sensor. The transmission shaft is equipped with a combined inertia plate group for adjusting an inertia load. A position sensor is arranged between the transmission gear set and the loading equipment. The data acquisition and analysis equipment is connected to the motor to be tested, the torque rotation speed sensor and the position sensor. According to the motor performance testing system, the high precision position detection can be provided, at the same time the inertia load can be adjusted, the simulation of various working conditions is facilitated, and the system is especially suitable for the motor tests of all working conditions.