Abstract: The paper introduced effect of oil temperature change on oil viscosity, hydraulic medium and rubber seal of hydraulic system of 40T support carrier, analyzed total heat power, total heat-dissipated power of the hydraulic system, and found relation between total efficiency and oil temperature change of the hydraulic system and gave its function relation by use of dynamic thermal equilibrium computation. According to the relation and sampling of measured oil temperature data of an operating mode, it calculated corresponding efficiency value, confirmed the relationship that oil temperature change causes efficiency change of the hydraulic system and obtained conclusion that total efficiency changes with oil temperature change, namely when difference in temperature increases, total efficiency reduces; when difference in temperature reduces, total efficiency increases. The system's working status could be grasped through the conclusion, and reducing oil temperature can raise efficiency effectively.