Permanent magnet linear machine (PMLM) can directly output power in linear motion, which has significant advantages of simple structure and high efficiency. It is suitable for some linear motion applications and attracts more and more attentions. The widely existing magnetic field modulation effect is analyzed, several typical topology structures of PMLM are taken as examples, and the application of the internal magnetic field modulation mechanism is pointed out. Meanwhile, in view of the special structure of PMLM, several modeling methods of electromagnetic parameters are discussed. Finally, the prospects of research on the magnetic field theory, topology structure and application of PMLM are analyzed, providing certain reference value for the practical engineering application of linear machines.