Abstract: A liquid cooling system of blade server is introduced in this article. The flow and the pipe diameter of various parts of the system are designed. And the performance of heat transfer is verified by simulation and experiments. The result shows that the liquid cooling solution can increase the power of the blade CPU (Central Processing Unit) from 165 W to 300 W compared with air cooling solution. And the performance of the liquid cooling system can be improved by using spade-tooth cooling plates and water refrigerant and controlling the flow below 1.5 L/min. The costs of liquid cooling and air cooling are compared. The payback time of liquid cooling system is about 2.5 years, so the liquid cooling system is more economical for long-term operation.