EMI of the switching-converter is decided by the spectrum of the PWM waveform. There is great theoretical and practical meaning to quantify and analysis them. This paper takes the PWM drive waveform as a time series of processes or events. Based on the analysis of the periodic frequency- spreading, statistical theory is applied to analyze the probability density function, characteristic function, invariable distribution and power spectral density of the PWM waveform under chaotic frequency-spreading, the characteristics of chaotic frequency-spreading are obtained. Finally, some feasible experiments are carried out which verified that the EMI of the switching converters is suppressed effectively by chaotic frequency-spreading.