In order to solve the problem of heat dissipation of the air-cooled laser whose output energy was not less than 220 mJ and whose repetition rate was 40 Hz, a 3-D model of the laser and the heat sink was designed and constructed, and thermal simulation software was used to conduct a thermal simulation of the heat dissipation of the laser. The analysis results were optimized and verified by experiments. The results show that when the ambient temperature is 55 ℃, the output energy of the laser is greater than 220 mJ, and the repetition frequency is 40 Hz, the finished laser with red copper fin radiator can meet the requirements of continuous operation, and the design allowance is about 15%. This thermal simulation method can match the actual situation well, and the research provides an effective reference for the further thermal design of the laser