Abstract: The non-sequential ray-tracing is introduced to the integrated opto-thermal-mechanical method for analyzing the thermal effect of optical system under complex light source condition. The finite element analysis is used to solve the temperature field distributions of the lens and the deformations of the optical surfaces. A basis function based on the Zernike polynomials is proposed to fit the gradient index field caused by inhomogeneous temperature. Then the optical performance of the system can be obtained with higher accuracy. Weights of the Zernike polynomials are used in fitting processing to avoid the asymmetry of the results. At last, a high power laser expander is analyzed as an example. The simulation results show that the method proposed is effective and generality.