Based on the thermal model of thermo-optic device and analytical expression of temperature distribution, by employing the ray optics, the beam expansion cause and law in thermo-optic-effect-induced total internal reflection (TIR) are analyzed by calculating the ray reflection trace in different meridan plane of the core, and a theoretical method to calculate the specific expanding value of beamwidth is given. A 2×2 polymer thermo-optic switch with high reflection coefficient is designed as an illustration of the law. Utilizing the law in authors′ design, the structure parameters of expansion waveguide width, buried depth of the core, and enlarged reflection area width are calculated. Simulation results show that the structure exhibits a reflection coefficient of 82.6 % at reflecting state, a cross coefficient of 84.4% at cross state, the cross talks are better than 30 dB. The simulation optimized result of these structure parameters agrees with theoretical calculation.