The flushing flow affects the cooling equilibrium of the pump motor in a closed hydraulic system.In order to ensure more balanced cooling of the pump motor, the method of determining flushing flow in a closed hydraulic system was studied.The results show that the flushing flow in the fixed displacement motor system and the variable displacement motor system is about 14.6% to 31.5% and 9.6% to 29.4% of the theoretical rated output flow of the slippage pump respectively, or about 2.88% to 7.39% and 1.88% to 6.89% of the theoretical rated output flow of the main pump respectively. In the initial design, the flushing flow of the fixed displacement motor system and the variable displacement motor system can be selected as 23% and 19.5% of the theoretical rated output flow of the slippage pump, or 5.1% and 4.4% of the theoretical rated output flow of the main pump respectively. The correctness of the method of determining the flushing flow was indirectly verified through the prototype testing.