This paper focuses on the optimal design of permanent magnet linear synchronous motor (PMLSM) with trapezoidal permanent magnets (TPMs) at multispeed that are applied in laser-engraving machines. Particle swarm optimization (PSO) and signal-to-noise (SN) ratio method are proposed to minimize the thrust ripple and maintain average thrust. First, an analytical model based on equivalent magnetic charge method is presented to analyze the PMLSM with TMPs, and the accuracy of the proposed analytical method is validated by finite-element method. Then, the PSO is applied to optimize the pole pitch (tau), air gap length (delta), bottom angle of TPM (theta), and magnet height (h) to obtain the optimized structure parameters at a single speed. Application of PSO is repeated at different speeds to obtain their optimal structure parameters. Finally, the SN ratio method is used to synthetically optimize the parameters obtained at different speeds to acquire the optimized parameters at multispeed. Experimental results of prototype can confirm the validity and superiority of the method.