Xenon lamp-pumped Ndglass will result in uneven temperature rise, which will produce thermal stress birefringence and induce depolarization. Depolarization will directly reduce the system efficiency, beam quality. So, it is important to confirm the temperature distribution and stress distribution, to accurately predict the resulting depolarization of the beam, and to design rational beam filling factor. Based on the electro-optical conversion model, we used three-dimensional ray tracing to obtain the distribution of thermal power source. with heat conduction theory, we calculated the three-dimensional temperature distribution of the laser glass and the stress distribution and the resulting depolarization distribution. The results show that the stress birefringence within the beam diameter is small, but the four corners of square beam department of the stress birefringence are more serious, with the largest depolarization about 0.13%. The results are consistent with the experimental results of Lawrence Livermore national laboratory. The near field results of outputted laser show that the thermal induced depolarization of the slab amplifiers satisfies the output design demand of high energy equipment.