Regulations in fuel consumption and emission become more and more strict, it is required to give more research on finding methods of saving energy and reducing emissions, also, reducing energy loss of engine attachments is an effective method. Oil pump play a key role in engine lubrication system, it provides power to make oil flow for lubrication and cooling of moving parts in engine. Due to discharge effect of the pressure relief valve in oil pump, thus, traditional oil pump loses a lot of energy as engine works in a high speed period. On the contrary, variable displacement oil pump has a certain effect on energy-saving by reducing oil pump displacement directly when an engine is running in a high speed period. This article gives research on the structural characters and energy-saving effect on both rotor type and vane type of variable displacement oil pump.