The hydraulic retarder is the most stabilized auxiliary braking system [1-2] of heavy-duty vehicles. When the hydraulic retarder is working during auxiliary braking, all of the braking energy is transferred into the thermal energy of the transmission medium of the working wheel. Theoretically, the residual heat-sinking capability of the engine could be used to cool down the transmission medium of the hydraulic retarder, in order to ensure the proper functioning of the hydraulic retarder. Never the less, the hydraulic retarder is always placed at the tailing head of the gearbox, far from the engine, long cooling circuits, which increases the risky leakage risk of the transmission medium. What's more, the development trend of heavy load and high speed vehicle directs the significant increase in the thermal load of the hydraulic retarder, which even higher than the engine power. Conventional engine cooling system could not meet the demand of the hydraulic retarder heat rejection within the same installation space.In this research, independent two-phase evaporator was adopted to strengthen the coolant heat absorption capacity from the transmission medium at the oil outlet of the retarder and increase the thermo stability of heavy-duty vehicles during high working load operating conditions by means of the vacuum flow boiling heat transfer [3,4,5]. As a result, the reliability during driving could be guaranteed.The basic idea of this study is determining the thermal load characteristics and the transmission medium circular flow by a heavy-duty vehicle operating conditions. Then, the geometric dimensions of the evaporator are confirmed according to the optimal temperature range of the transmission medium. Finally, analysis is conducted for the influence of the coolant flow variation on evaporation system heat transfer power under different operating conditions. The mathematical model of the coolant vapor fraction, the flow and the oil temperature is obtained.The result shows that under different operating conditions, the designed two-phase evaporation system could stabilize the transmission fluid in the best operating temperature ranges. On the premise of the system's stability, the cooling medium's vapor fraction variation could dynamically respond to the change of vehicle driving conditions. Moreover, this mediates the variation of the recovery flow in the cooling medium and reduces the volume flow significantly. Above all, this improves the controllability of the two-phase evaporation system and decreases energy consumption of this system in the meantime.