The working procession of engine retarder was discussed. With a diesel engine as an example, the mathematical model of compression release engine retarder was created. The influence of release valve working parameters on the braking performance was analyzed by the method of numerical simulation. The simulation results showed that there was a crank angle before the top dead center to one engine speed specified to get the maximum brake power, this angle was bigger when the engine speed was higher; and the higher release valve lift or the faster release valve velocity was, the more brake power could be gotten.