In order to optimize the performance of engine bleeding brake, a three-dimensional model was built with UG. The model was divided into grid by the dynamic mesh technique. Besides, the motion rule of the intake valve and piston was set and defined. By applying the three-dimensional transient numerical simulation in the process of engine brake working by Fluent, the changes of pition pressure were analyzed. According to the piston's instantaneous force, the engine average braking power in different conditions was calculated, and then the influence of exhaust valve lash, engine speed and exhaust back pressure on engine braking performance was also obtained. By optimizing working parameters, the performance of engine bleeding brake was enhanced, and the simulation results were verified with experiments. The results have shown that the maximum error is 5.2%, and the smallest error is only 4.1%. It is shown that the simulation results are of high precision and accuracy.