Abstract: Abstract: Based on the layout principles of inlet and outlet pipes, using the cavity acoustic modes to design muffler can get good acoustic attenuation, and for the inlet and outlet pipes are not coaxial, the pressure loss is larger when it is caused by the blocked air flow. Taking the expansion chamber and flow-reversing chamber as examples, firstly, comparative study about the effects of chamfer transition and circular arc transition on the pressure loss and transmission loss were made based on the finite element method. The results showed that the smooth transition of outlet pipe could decrease the pressure loss and did not affect the transmission loss at the same time; moreover, the properties of muffler with circular arc transition were much better than those of chamfer transition. On this basis, the coupling influences of transition circular arc radius and muffler size on the pressure loss and transmission loss were analyzed. It turned out that the transition circular arc of outlet pipe almost had no effect on the transmission loss but could effectively decrease pressure loss; moreover, this is a universal law and will not be changed with the change of type and size of the muffler. And with the increase of transition circular arc radius, the change rates of the pressure loss were decreasing. Further, under the premise of keeping the overall size, an automobile muffler was made a preliminary improvement by applying the layout principles of inlet and outlet pipes based on the cavity acoustic modes. After the preliminary improvement, the average transmission loss was increased by 15.8 dB in the frequency range of 750-1500 Hz, but the pressure loss was increased by 7.78% with the inlet velocity of 60 m/s. In order to decrease the pressure loss on the base of good acoustic attenuation performance, the further improvement of car muffler was made. The first chamber and third chamber of muffler could be seen as two simple flow-reversing mufflers, and 10 mm transition circular arcs were added at the entrance of outlet pipes on the basis of the preliminary improvement. Compared with the original muffler, the average transmission loss of final improved muffler in the frequency range of 750-1500 Hz was increased by 15.3 dB and slightly lower than that of preliminary improved muffler, however, the pressure loss was reduced by 25.20% with the inlet velocity of 60 m/s. In conclusion, the properties of muffler have been greatly improved after the further improvement. Integrated use of the layout principle of inlet and outlet based on the cavity acoustic modes and the effect laws of outlet pipe transition circular arc on the pressure loss can guide the muffler design.