As a command carrier of the motordriven system, signal integrity of PWM has impacts on reliability and ability to fulfill tasks for the whole system. However, there few research report at home and abroad. A newtype circuit of fault protection and diagnosis is proposed for improving the signal integrity of PWM during transmission. A series of logical operations are to deal with PWM from DSP and signals modulated by drivers in this circuit. And decision thresholds are set up by hardware circuits, the PWM output is inhibited when noncoincidence value between the two signals is larger than decision thresholds. It is verified by experiments that, this circuit can be able to diagnose faults in time, and parts of fault can be insulated, if a sudden change or big delay occurs during PWM transmission. So that it plays a protective role for power devices, and the reliability of the motordriven system is improved.