Vehicle hydraulic retarders are applied in heavy-duty trucks and buses as an auxiliary braking device. In traditional cooling systems of hydraulic retarders, the working fluid is introduced into the heat exchanger to transfer heat to the cooling liquid in circulation, whose heat is then dissipated by the engine cooling system. This prevents the waste heat of the working fluid from being used effectively.In hydraulic retarder cooling system based on the Organic Rankine Cycle, the organic working fluid first transfers heat with the hydraulic retarder working fluid in Rankine cycle, and then outputs power through expansion machine. It can both reduce heat load of the engine cooling system, and enhance thermal stability of the hydraulic retarder while recovering and utilizing braking energy.First of all, according to the target vehicle model, hydraulic retarder cooling system model based on Rankine cycle is established. Through presetting Rankine cycle parameters, characteristics and power consumption of this cooling system are analyzed under different hydraulic retarder working conditions. Then a mathematical model of traditional hydraulic retarder cooling system is set up. Its characteristics and power consumption under different hydraulic retarder working conditions are analyzed. Next, the effectiveness of the mathematical model of hydraulic retarder cooling system model based on Rankine cycle is verified by experiments. Finally, based on the Rankine cycle model after verification, two cooling systems are compared in cooling capacity, power consumption, economy and volume under the same circumstances.The results show that, in contrast to traditional cooling system of hydraulic retarder, hydraulic retarder cooling system based on the Organic Rankine Cycle, though taking up a little more space, can effectively reduce heat load of engine cooling system, improve thermal stability of hydraulic retarder, lower energy consumption and be more economical.