Aiming at the plasma shock wave effect produced by the surface gear material 18Cr2Ni4WA in the femtosecond laser micromachining process, the pressure equation of the plasma shock wave propagation and the wavefront temperature model of the micromachining material are established. Using pulse width, energy and defocus as the changing parameters, the law of shock wave propagation radius with temperature and pressure is obtained. Experiments have verified that when the femtosecond laser pulse width is 300, 500, 800 fs, the single-pulse laser energy is maintained at 15-25 μJ. Meanwhile, the molten layer is smooth and flat, which has a good ablation effect. Under the same pulse width, excessively high energy will cause too much melted material, and the depth of the pit will deepen. As a result, the discharge effect will be reduced, and uneven wave crests will be formed on the pit wall. When using higher energy parameters to ablate the material with variable defocus amount, as the positive defocus amount increases, the edge heat-affected area increases. At the same time, the discharge effect of the plasma shock wave on the liquid material is reduced due to the defocusing change. The plasma shock wave effect model was verified, and the actual ablation effect was achieved.