Abstract: The traditional measurement of the transmission loss of muffler was done in single terminal boundary conditions, the sound fields at the upstream side and downstream side of the muffler were considered to be two independent acoustic systems. Then the transmission loss of muffle was determined by calculating the incidence power of upstream side and the transmitted power of downstream side respectively. But it failed to take into account the full coupling effect on the sound field of upstream side for the reflected wave from the terminal of the downstream side of the muffler, limiting the measurement accuracy of the sound transmission loss. Consequently, in order to take into account the coupling fully, the new measurement method of transmission loss, which based on transfer function, 4 microphone locations, and dual terminal boundary conditions, was put forward. Furthermore, transfer function correction algorithm which can improve the accuracy of transfer function measurements also was derived. In the end, the experiment for transmission loss of a typical expansion muffler using hardware platform of B&K acoustical material testing system was done, and the results were consistent with the theory.