Based on heat conduction theory, the heat conduction equation of slab and bulk crystals is established. Making solution for heat conduct equation and numerical simulation, the temperature distribution, thermal stress and thermal lens effect of the Tm:YLF of slab and bulk crystals is analyzed comparatively in the condition of high pumping power. The calculated results indicate that the slab crystal is more suitable for the output of high power laser. The necessary theoretical guidance is provided for developing the laser power.