This paper focuses on the thrust ripple of a permanent magnet linear synchronous motor (PMLSM). The PMLSMs with the North-South (NS) and consequent (C) poles are compared first. Then two magnetic regulating structures whose widths are tau and tau / 3 (tau is the pole pitch) are proposed and set on the primaries, formingModulars I and II structures, respectively. The effects that the modular structures exerting on the detent force and voltage harmonics are expounded. At last the modular primaries are selectively applied to the PMLSMs with different poles: the modular I whose magnetic regulating structure employs magnetic permeable material is applied to the C-pole PMLSM, and thus the thrust ripple caused by the voltage harmonics is suppressed remarkably. Whereas the magnetic regulating structure of the modular II which is nonmagnetic is used for the NS-pole PMLSM, helping to reduce the detent force and improve the average thrust.