A variety of water cooling designs for conduction cooling end pumped slab (CCEPS) laser are simulated with computational fluid dynamics (CFD) methods. The difference between fluid-solid coupled method and mere heat conduction simulation in gain media is comparatively investigated. The influences of the size, number of the cooling channels and flow rate of the cooling water to the temperature distribution of the slab and the flow resistance characteristics of the heat sink are studied. In general, decreasing of characteristic size, increasing of channel numbers and the flow rate of the cooling water can reduce the thermal resistance between the solid and fluid interface. Thus total heat transfer coefficient is extended to a very high level. That is, total thermal resistance of heat sink can be remarkably reduced with micro-channels cooling structure compared to cavity or mini-channels structure. The temperature level of gain media can be apparently lowered down. However, the pressure loss in heat sink increases significantly.