The principle of field synergy is to simulate the optimization cooling fan position for CPU of desk computer in this study. The parameters U, V, W and input position of fan are calculated to simulate cooling capacity of CPU for desk computer. Temperature and velocity fields are simulated by COMSOL Multiphysics software. Cooling capacity is calculated by field synergy mean square root method in this research. The simulation result shows that field synergy angle decreases while cooling capacity increases. It is very obvious that the better input position of fan leads lower field synergy angle.