This paper aims to develop a stable and accurate modeling method for thermally induced refractive-index fields in lenses under non-axisymmetric heating conditions. The target problem is the transformation of discrete three-dimensional temperature data from finite element analysis into a refractive-index model that can be directly loaded into optical design software for ray tracing and image-quality evaluation. To address the contradiction between field representation accuracy and numerical robustness in ray tracing， a segmented analytical strategy is established for complex off-axis thermal conditions. The method is further verified on a stealth dicing laser head so that the influence of thermally induced refractive-index variation on wavefront quality and focus position can be quantitatively predicted within an optothermal integrated analysis framework.A layered Gaussian regression scheme was proposed to represent the non-axisymmetric internal temperature field of a lens. Along the optical axis， the effective axial interval of the lens was divided into K non-overlapping layers. Within each layer， the transverse temperature distribution on the plane perpendicular to the optical axis was fitted by a two-dimensional Gaussian function with offset center， amplitude， spread parameters， and background term. Based on the material thermo-optic relation， the fitted temperature rise in each layer was converted into the corresponding refractive-index increment， and a piecewise refractive-index field was constructed. To improve tracing stability， the axial variation was represented by parameter switching between adjacent layers， while the axial refractive-index gradient inside each layer was approximated as zero. A user-defined dynamic link library was then developed to return the refractive index and its spatial derivatives at arbitrary coordinates during optical tracing. For validation， an optothermal integrated simulation was carried out for a stealth dicing laser head with focal length 9 mm， wavelength 1099 nm， numerical aperture 0.83， and maximum half field angle about 1°. Under a 20 W Gaussian beam with 1° tilted off-axis incidence， the absorbed power obtained from non-sequential ray tracing was imported into the finite element model as a volumetric heat source， and the resulting temperature data of lenses L1 to L8 were used for fitting， model loading， and optical evaluation.The proposed method reproduced the three-dimensional temperature fields of all lenses with high fidelity. As the number of layers increased， the fitting root mean square error decreased significantly and became stable after K≥7， indicating convergence of the layered approximation. Considering both computational efficiency and fitting accuracy， K=10 was selected for subsequent analysis. For lenses L1 to L8， the coefficient of determination ranged from 0.9939 to 0.9999， the RMSE remained below 0.02 °C， and the normalized RMSE was within 0.0033 to 0.0225， confirming that the method can accurately characterize complex non-axisymmetric temperature distributions. After the fitted parameters were loaded into the DLL， the refractive-index field of the lens could be reconstructed in optical software in real time. Numerical stability analysis using lens L7 showed that when K≥9， the system wavefront RMS and P-V no longer changed noticeably with increasing layer number. Under the representative 20 W and 1° off-axis incidence condition， the RMS of the thermally induced wavefront difference was 0.0212λ. Although the heating condition was non-axisymmetric， the induced aberration was still dominated by defocus. The focus-shift analysis based on RMS spot radius versus defocus further showed that， after the system reached steady state， the best image plane shifted by about 0.16 μm along the negative optical-axis direction. These results verify that the proposed method can complete the prediction chain from finite element temperature field to refractive-index field and then to optical-performance metrics.The layered refractive-index modeling method provides a practical interface between non-axisymmetric thermal analysis and optical performance evaluation. It can transform discrete thermal results into a queryable analytical refractive-index model with sufficient fitting accuracy and reliable tracing stability. The method is suitable for optothermal integrated analysis of high-power laser systems under asymmetric thermal loads. Because the in-layer basis function is replaceable， the framework also has good extensibility for other temperature-field morphologies and more complicated engineering conditions.