In a high-power optical cutting system, the laser is focused by the optical system to generate heat, which causes the optical system components to undergo thermal deformation and change in refractive index, which changes the focal length of the lens and affects the processing effect. COMSOL software is used to model the optical system in a multi-physics field, the shape of the mirror and the change of refractive index under the action of continuous laser and quasi-continuous laser are obtained by simulation. The heated data of the lens is imported into ZEMAX for beam tracing, and the axial offset of the focal point of the optical system is calculated by simulation. The higher the power, the greater the focal shift; the larger the pulse width, the smaller the focal shift; the higher the repetition frequency, the smaller the focal shift. Finally, the focus shift is compensated by introducing the convection coefficient, and the position of the focus can be controlled by changing the convection coefficient. This research solves the problem of difficult measurement of the axial focus offset during high-power laser processing and provides a theoretical basis for the focus control of high-power laser cutting equipment.