A brushless DC motor driving system, in which the duty cycle of the PWM control signals for the duty cycle generator is determined by a speed control circuit, a feedback comparator, and the set and reset signals of a S-R Flip-Flop, wherein the PWM control signals pass through the Hall synthesis circuit, the logic control circuit to drive the coil by the drive current generated, and the duty cycle of the PWM control signals is further adjusted by means of the feedback sensing voltage obtained from the driving current and sensing resistance and fed back to the feedback comparator, the stability and linear stability of the input voltage and the driving current being achieved.