In this paper, we present a general design methodology for single-feed multi-beam transmitarray antennas by using a parallel particle swarm optimization (PSO) algorithm. The conventional PSO method is reviewed and a modified PSO algorithm is proposed which can be applied in parallel computing. Then the proposed method is implemented for a triple-beam transmitarray design. The numerical results show that the parallel PSO method contributes to a better radiation performance and reduces 87.6% of the computation time compared to the conventional PSO method.