For a laser with stable performance, the entire system is in a stable state of heat exchange during the working status, that is, the laser wavefront aberration can remain stable and unchanged. Based on this feature, the fabrication of free-form mirrors is proposed to correct the wavefront aberration of the laser and improve the beam quality. Firstly, the wavefront compensation principle, design and manufacturing process of the free-form surface mirror are introduced, then its wavefront compensation ability is theoretically simulated through mathematical modeling, and finally the wavefront compensation experiment is carried out in the slab solid laser. The result is that the free-form surface mirror improves the laser beam phase and far-field beam quality at an output power of 12.5 kW. Unlike anamorphic mirrors, free-form mirrors are not constrained by actuator spacing, and are therefore more effective at correcting higher-order aberrations, are less expensive, and do not add to the complexity of the system, each of which has its own advantages over an adaptive optics system.