In order to improve output waveform quality of cascade STATCOM by ladder wave modulation method, put forward conduction angle optimization method with total harmonic distortion rate (total harmonic distortion, THD) as direct optimization target, through the particle swarm optimization (particle swarm optimization, PSO) algorithm optimizing objective function of conduction angle derived in this paper. Numerical experiment results show that the proposed PSO-THD cascade STATCOM staircase optimization algorithm has smaller THD of output line voltage than traditional optimization algorithm.