With the improvement of power, efficiency, reliability, manufacturability, and cost of high power semiconductor laser, many new applications are being enabled. Most of the semiconductor laser bars are packaged with the indium solder. However, some small voids are created during the packaging process, which will be gradually enlarged by the electromigration and electrothermal migration of the indium solder. Voids may cause local overheating near the facets of the laser. Therefore it is necessary to study the thermal behavior of semiconductor laser bars with voids in the solder layer. The thermal behavior of a single-bar CS-packaged 40 W 808 nm semiconductor laser with voids in the solder layer is studied and the relationship between temperature and voids size is analysed. The distribution of voids is predicted according to the space spectrum of a 40 W 808 nm semiconductor laser bar and the simulation results. It is found that the simulation results agree well with the measurement of the scanning acoustic microscope (SAM) image of solder layer.