In order to improve the measurement efficiency of symmetric muffler transmission loss, we explore the mutual correspondence between incident sound wave and reflected sound wave in the upstream and downstream pipelines of different sound source positions, and propose a single load transmission loss calculation model suitable for various types of mufflers, based on the theoretical analysis of two-source method. To decrease the difference between the calculated result and actual measurement result which caused by the large reflection coefficient at the sound absorption end, a modified formula has been derived to eliminate the multiple reflections of the transmitted reflection sound wave at the end of the test pipe. Three typical mufflers have been tested by self-made impedance tube. The results show that the single load calculation model can accurately calculate the transfer loss of the symmetric muffler when the sound absorption performance of the acoustic load at the end is good. The modified formula can effectively eliminate the influence of the reflected wave caused by the acoustic terminal load on the calculation model of the single-load transmission loss, can reduce the requirement on the acoustic absorption performance of the terminal acoustic load, and ensure the applicability of the calculation model of the single load transmission loss.