Thermal design is the key technology for a diode pump solid laser to keep high stability of the output energy. To solve the cooling problem of a solid laser, a 150mJ air cooled YAG laser with the repetition of 50Hz was systematically studied. The 3-D theoretical model of the diode-pumped laser and the cooling parts was constructed, and the generated heat of the laser was calculated. Further, the thermal management of the whole system was analyzed using the software of FloEFD, and the theoretical results were then discussed. Finally, the verified experiment was carried out. The results show that after three cycles, the laser output is 155mJ, 50Hz, the laser beam dispersion is 2.9mard, and the temperature of radiator is about 85℃, respectively. The thermal design in this study has high stability, with which the cooling problem of the laser can be perfectly solved to meet the final requirements of the system and guarantee the natural work of the laser. This study provides a reference for the further thermal design of laser.