Aiming at the exhaust muffler of a 244S motorcar, the acoustics field model and the flow field model are established by adopting Finite Element Method. The 3-D acoustic simulation is carried out by using software SYSNOISE, and an improvement scheme with optimized inner structure of the muffler is proposed based on the simulation. The acoustic simulation result and the bench test result of the improved muffler show that the muffling ability in middle and high-frequency of the improved structure is obviously stronger than the original structure, and the simulation result coincides well with the bench test result. Meanwhile, the flow field simulation result shows the power loss of the engine has little change, which means that the structure improvement of the original muffler is effective.