Chaotic SPWM control has attracted much interests due to its effectiveness for EMI suppression in power converters. However, most researches focus on the simulation and experiment of power converter under chaotic SPWM control, which is lacking a quantitative method. Based on double Fourier series this paper provides a spectrum calculation method for multi-period SPWM or quasi-random SPWM signals firstly, and the related spectrum calculation and simulation for multi-period SPWM are given to verify the accuracy of the spectrum calculation method; then the calculation method is extended to the spectral analysis of chaotic SPWM signals. To observe the impact on the spectrum of chaotic SPWM signals generated by different mappings and in different variation ranges of carrier period, a spectrum comparison between the Tent and Chebyshev mappings is conducted, in which results indicate that the variation range of the carrier period and the selection of mappings have a great influence on spectrum distribution; in the long term, probability density distribution of chaotic mapping will certainly affect the spectrum, and in the short term the initial value of the mapping will also affect the spread spectrum distribution. In summary, the proposed spectrum calculation method in this paper provides a theoretical foundation for the spread spectrum principle of chaotic SPWM control and for the design reference in practical engineering application.