By using a Simulated model, this article established the basic heat transfer equations for laser-gas cooling systems. It can be used in the determination of heat resistance of each part in the system. Analysis indicates that increase in the circulating gas density can improve the heat transfer effect, decrease gas flow resistance and reduce power consumption. On this basis a medium pressure gas cooling system was built and succeasfuly used in a YAG laser which has a repetition rate of 10 Hz and heat dissipation power of 200 W.