In closed hydraulic system，charge parameters calculation is closely related with cooling requirement.System heating power and oil cooler cooling power were calculated by system total power loss.System heating power is about 18.6%~22% of system input power,and oil cooler cooling power is about 22.3%~26.4% of system input power.Power loss ratio between pump and motor is about 0.93~1.35.For system overall heat balance consideration,charge flow has a linear relationship with system input power.Charge oil ratio between pump and motor is about 0.93~1.35.It can be designed as 1.14,and in practical application it can be adjusted by changing flushing valve of motor.Further,the displacement of charge pump is about 19.63%~23.45% of main pump displacement when full displacement system pressure is matched at 15 ~20 MPa.System cooling requirement and charge parameters were designed by this method and checked on prototype.The test results show that the system has a good heat balance.