Abstract: Mufflers in practical engineering applications often contain complex internal components and the internal flow field is distributed unevenly. In addition, the flow-reversing pipes and perforated elements in the muffler further complicate the flow patterns. Therefore, the effect of non-uniform flow on acoustic attenuation performance of mufflers should be carefully studied. In order to calculate the transmission loss (TL) of muffler, the computational fluid dynamics (CFD) software FLUENT is used to capture the inner flow field, and then the fluid property data are imported to the acoustic finite element model of LMS Virtual Lab. The different perforated impedance models are adopted to calculate TL, and the numerical results are compared with the results of previously published experimental measurements. The characteristics of the flow inside muffler are studied in detail. Furthermore, the effect of flow velocity on the acoustic attenuation performance of muffler is investigated. It is concluded that for the studied muffler, increasing flow velocity raises the TL and lowers the resonance peaks.