To prevent engine coolant temperature fluctuation and insufficient water supply in an eddy current dynamometer during engine bench tests， a second stage cooling system was added to engine pilot as a solution. First， a simulation of the system was performed with the Flowmaster based on relevant parameters to optimize the system， and then a comparison experiment was conducted to verify the simulation. The results show that the temperature of the engine coolant can be stable within±3 ℃， water flow rate in the dynamometer is 4 m3/h more than before.