A frequency jitter generator and a frequency jitter PWM controller are provided for overcoming the shortcoming that a conventional PWM controller reduces the electromagnetic interference issue by means of varying the operating frequency of the PWM controller based on an input voltage, while resulting in the uncertainty of the range of frequency jitter and the difficulty circuit design due to the effect of the input voltage and the load. The frequency jitter generator and PWM controller adjust the range of frequency jitter by using a signal within a fixed voltage range. The invention not only gets rid of the effect of the input voltage and the loading, but also simplifies the circuit design by fixing the range of frequency jitter no greater than a predetermined percentage regardless of the operating frequency of the PWM controller.