Through the analysis of the process that closed hydraulic system generating heat internally, and its oil absorbing and radiating heat, a schematic of the heat transfer was drawn, and the intern oil temperature calculation formula of closed hydraulic system was listed based on the theories of conservation of energy and thermodynamics. A method was proposed that used the summation of leakage flow and flushing flow to calculate the total amount of cool oil supplemented into the system, and the condition of oil temperature in closed hydraulic system of WC80Y continuous mining truck was calculated through analysis. The results show that the oil temperature of the inside closed hydraulic system has a relation with system’s overall efficiency, system oil charging and flushing capabilities, work pressure and radiator efficiency, and other various factors. The validity of this method is verified through examples, and the overheat problem of its oil is solved, which has a reference effect on the design of closed hydraulic drive system for similar large scale walking construction vehicles.