A thermal conduction model of cylindrical laser crystals was established. The temperature distribution in the laser crystals with different Nd concentration and different pumping radii were obtained by using the Poisson equation. The study result shows that with the increase of Nd ion concentration, the central temperature at the end of the ND : YAG crystal will rise and the temperature at the central axis of the crystal will decrease under the same condition. For the same crystal, when the pumping power is given, the central temperature of the Nd : YAG crystal will rise. If a lower ion doped crystal is used as the laser midium and the pumping radius is increased rationlly, the thermal effect of the crystal can be restained. The conclusion has provided a theoretical direction for the improvement of the stability of lasers and the study of the thermal effect of crystals.