In order to investigate the decompression engine brake performance,a mesh model is established withthe help of the software CFD (Computational Fluid Dynamics) according to the analysis on its working process.Then,a 3D simulation of the working process of the transient decompression engine brake is performed by means ofthe dynamic mesh technology,and the simulation results are verified by experiments.The results show that (1) thehigher the rotating speed of engine is,the better its braking performance will be; (2) the larger the decompressionvalve opening is,the higher the brake power peak will be; (3) however,the maximum decompression valve openingis limited by compression clearances and piston movement rules; and (4) at a certain rotating speed,each decom-pression valve opening corresponds to an optimal decompression valve advance angle,which causes an optimal bra-king performance.Besides,the optimal valve advance angle decreases as the decompression valve opening increases.