Abstract: For studying the different structures of muffler on the aerodynamics effects，the fluid mechanics analysis software FLUENT was used to simulate the states of automobile muffler inner flow and do the numerical simulation.Effects of expansion ratio and inlet velocity on loss of gas pressure in the muffler were analyzed，and flow rate-pressure loss curve in figure had been get.Numerical simulation showed that depth and its relative position of insert-pipe had a great influence on the muffler pressure loss.Numerical analysis of flow field had been confirmed validity.The study played a guiding role in improving the muffler′aerodynamic characteristics.