This paper discusses the energy saving principle of frequency conversion chilled water system. Analyzes the energy saving failure reason in actual engineering from the pipe lines and the pump two aspects and gets out the reason is adopting the constant pressure difference control method. Through a simplified chilled water system, compares the pump energy saving efficiencies under constant and variable pressure difference control methods. Get out that the frequency conversion system can reach the maximum energy saving efficiency under variable pressure difference control method. And put forward that the operable minimum pressure difference control method can be adopted in actual engineering.