The converter is described as a continuous-discrete system based on hybrid system theory. According to the theory of chaotic PWM, chaotic point process model of converter is established. Chaotic PWM control is done with piece-wise chaotic maps. The characteristic function and distribution of chaotic PWM are obtained, the spectrum of chaotic PWM and periodic PWM are obtained too. Experimental results verify that the peak spectrum of chaotic converter can be suppressed with stable output, and EMI is improved distinctly.