Abstract: Abstract: Transmission loss is a key factor to evaluate the performance of micro-perforated mufflers. In actual application, the mufflers are always applied in the air flow circumstance which would change the acoustic and aerodynamic performance of micro-perforated mufflers. So it is necessary to study the effect on acoustic properties caused by the flow. In this article, the uniform flow is taken into account, which has influence on acoustic impedance of the micro-perforated panel and the sound propagation in the center tube.The theory that how normal incident flow and grazing flow affect the sound propagation is discussed. When it comes to the circular micro-perforated duct mufflers, the grazing flow plays a much more important role in the sound propagation than the normal incident flow. The reason is that only little normal incident flow could pass through the holes to flow into the expansion chamber. The authors establish the one-dimension transmission loss (TL) model of micro-perforated mufflers with uniformly grazing flow. The new model is verified by the experimental result. In the comparison with the original TL model without taking the flow into account, the TL curves calculated by the new model are consistent with the experimental curves, and the relative error ratio drasticallydecreases to 15% from 27.3%. So the new model we established in this article is more accurate. Furthermore, we analyzehow structural parameters exert impact on the acoustic performance of micro-perforated mufflers with uniformly grazing flow based on this model. When the direction of the grazing flow is the same with that of sound propagation, with the grazing flow speed increasing, the TL increases slightly in the low-frequency domain. However, the acoustic performance of the micro-perforated mufflers would degenerate a lot in the medium-to-high frequency domain.The research reveals that the influence caused by the main external size parameters of micro-perforated mufflers with uniformly grazing flow is almost the same with that without flow. As for the perforated parameters, they make a big difference on acoustic performance without uniformly grazing flow, which is different from that with uniformly grazing flow.The amplitude of TL will increase as the diameter of perforation and the thickness of perforated walls increase. And the different perforation ration will change the resonant frequency of the micro-perforated mufflers, furthermore, it also will make a big difference on the bandwidth of noise elimination. But the difference with the uniformly grazing flow is much less than that without flow.Changing the perforation parameters in the normal range, the difference of TL without flow will drastically reach 18 dB. In conclusion, designing micro-perforated mufflers with multi-cavities in the uniformly grazing flow circumstance only needs to determine the size of external structure according to the TL calculated under the condition which is without flow. As for the perforation parameters, we can set the values by experience because of their slight influence on the TL.The theoretical model in this paper can predict the transmission loss more accurately. And the study of the influence on acoustic performance caused by changing structural parameters can provide the reference to design a muffler according to the target frequency.