In this paper, a complicated resistance muffler was designed for a construction machinery. Computational Fluid Dynamics (CFD) method with three dimension models was used to explore the aerodynamic performance of the muffler. With this method the pressure distribution in the muffler is simulated and the pressure loss is predicted for the structure modification. The experiment results verify that the assembly performance of the muffler modified is better than the original muffler. The pressure loss and the insert loss of the improved muffler meet the design specifications and standards of the construction machinary. The integrated performance of the construction machinery has been advanced.