Gain medium of modern high-average-power solid-state lasers can produce distortion for the reason of thermal effect, such as YAG. Wavefront distortion and depolarization often occur simutaneously. The thermal stress of the high-power solid state laser slab is becoming a serious problem for hindering the further power increasing of the laser. A method for calculating the thermally induced refraction index in a pumped heat capacity slab laser was studied. In this method, the fourth-rank piezo-optics tensors of YAG crystal were transformed from the crystal lattice coordinate system to the laboratory coordinate system. With these new tensors, the thermally induced stress birefringence in a solid-state laser can be analyzed easily for any stress distribution. Furthermore, the calculations show that the birefringence in a zigzag slab depends on the angle under which the slab is cut from the boule. A theoretical description of the thermal and stress characteristics of heat capacity slab laser in two dimensional was also presented.