Optical structure–thermal simulation analysis is an effective method for predicting optical properties and optimizing optical systems. In this study, a multi-physical field coupling modeling method based on the COMSOL multiphysics finite element analysis software, coupled with heat transfer, solid mechanics, and geometric optics, is proposed. Compared with the traditional structure-thermal-optical property simulation analysis method, this method does not require surface fitting for optical lenses or data transmission through multiple software, improving the efficiency and accuracy of the simulation. A multi-physical field-coupling simulation analysis model is constructed for an off-axis four-mirror optical system. The structural and optical mirror deformations of the optical system under different temperature conditions are analyzed, and the optical performance changes are determined using ray tracing and spot diagrams, resulting in effective optimization of the optical system.