The thermodynamic model of laser medium was established to study the thermodynamic characteristic in laser medium cooling of heat capacity laser. The exchange heat on the end surface of laser medium was regarded as the heat source of the transient heat conduction equation. The heat conduction model in cooling was got. The transient temperature field in the laser medium cooling was got. Then, the thermal stress of the laser medium was got. By using numerical simulation, the cooling time, and the different in temperature and thermal stress of the YAG medium and the GGG medium at different cooling condition were obtained. The cooling time is greatly decreased as the convective heat transfer coefficient is increased from 0.1 W·cm-2·K-1 to 0.5 W·cm-2·K-1. The cooling time is little decreased as the convective heat transfer coefficient increased from 0.5 W·cm-2·K-1 to 1 W·cm-2·K-1. The cooling time of YAG medium is less than that of GGG medium at the same volume and the same initial temperture field. The difference in temperatue and the maximal equivalent stress of YAG medium are less than that of GGG medium′ at the same cooling condition.