A weight loading system for fitness equipments, comprising: a fitness equipment support, a rally rope, a motor, a motor driving module and a motor control unit, wherein the motor is a permanent magnet brushless DC motor with an external rotor structure, and the motor includes a stator mounted onto the fitness equipment support and an external rotor sheathed on the stator and rotatable with respect to the stator, and the rally rope is wound around the external rotor of the motor, and an end of the rally rope is coupled to the external rotor of the motor, and the other end of the rally rope is coupled to the power driving mechanism of the fitness equipment; the power driving mechanism is provided for driving the external rotor of the motor through the rally rope to rotate and generate a rally, and the motor control unit controls the motor to rotate in an opposite direction through the motor driving module, so as to produce a load force opposite to the rally, wherein the weight loading system further comprise an operation interface module, a torque control unit, a current regulating unit, a position sensing circuit and a power switch module; the motor driving module comprises a synchronous PWM generating unit, wherein the position sensing circuit is provided for sensing a rotating position and a rotating speed of the external rotor to determine whether the rally rope is situated in a stretched status or a retracted status, and the motor control unit includes a memory unit for recording an initial position of the external rotor to control the external rotor to return to its initial position; the operation interface module is provided for inputting an analog electromagnetic force Fe, such that when the tension sensor detects the rally Fc produced by the power driving mechanism currently, the motor control unit regulates the current of the motor according to the difference of the analog electromagnetic force and the rally Fe-Fc by the torque control unit and the current regulating unit, and a control module of the motor controls transmitting the current information and switch status module to the power switch module by the synchronous PWM generating unit to drive the motor to work or generate electric power; wherein the permanent magnet brushless DC motor includes a wheel hub comprised of a plurality of permanent magnets; the position sensing circuit is provided for detecting the position of the motor, a motor rotating speed calculation unit is provided to obtain the rotating speed of the motor, a corresponding speed compensation parameter is calculated according to the current rotating speed of the motor; the permanent magnet brushless DC motor has a three-phase winding, a Hall sensor is installed inside the motor, and a current conversion calculation unit is provided for providing a signal while the current is converted; the signal is combined with corresponding parameters, and the synchronous PWM generating unit generates six complementary three-phase PWM waveforms, and a driving signal isolation circuit increments the waveforms by 1, and performs a high/low voltage isolation, and then transmits the waveforms to a three-phase voltage inverter circuit to drive the permanent magnet brushless motor; a phase current sampling circuit is provided for detecting the current motor current and feeding back the current to the motor control unit; the phase current sampling circuit is a part of a motor protection circuit, wherein if the current exceeds a specific threshold, an alarm is alerted or the operation of the motor is stopped; a temperature signal processing circuit is provided for protecting a high power MOS tube in the three-phase voltage inverter circuit, to prevent the tube from being overheated or disabled.