AC charging spot communicates with electric vehicles through PWM signal, electric vehicles control the beginning and end of charging, and adjustment of charging current via detection of PWM signal statue. The accuracy and precision of PWM signal affect the whole charging process directly. GB/T 18487.1 has specific requirements about the parameters of PWM signal, however, unqualified PWM waves designed by different EVSE manufactures are generally found during the test. This paper analyzes the main causes of unqualified parameters of PWM signal based on the fundamental principle of the generation of PWM wave, and it also gives significance suggestions for the design and manufacture of AC charging spot.