In view of the harmonic distortion of Buck-Boost matrix converter (BBMC) output voltage waveform was directly related to its main circuit parameters, the Particle Swarm Optimization (PSO) was used to optimize its main circuit parameters. The basis of PSO was explained, and the method of optimizing its main circuit parameters by PSO was elaborated. Its optimization effect was tested by using Matlab/Simulink. Results show that compared with traditional method, main circuit parameters obtaine by PSO have lower harmonic distortion of output voltage waveform, and thus it has the certain application value.