Water muffler can effectively attenuate the noise of the pipeline system. In order to calculate the transmission loss (TL) of the water muffler accurately, the acoustic field of the water muffler was analysed based on the acoustic solid interaction model. The acoustic solid interaction effect of different structural parts on the acoustic performance of the expansion chamber muffler filled with water was studied. The TL of the water muffler was predicted with the ratio of elastic wall thickness to radius which equals to 1. The obtained results were compared with the analytical solutions and the numerical results of the case with rigid wall, which is used to validate the accuracy of the adopted acoustic solid interaction approach. On the other hand, the results based on the two dimensional axisymmetric model and the complete model were compared to validate the feasibility of the present method. The numerical results show that the decrease of the water muffler's structure thickness caused the enhancement of interaction between the solid and the water, leading to obvious effects of the elastic wall on the muffler's acoustic characteristics. The interaction between the circumferential wall and the water made the TL curve of the expansion chamber move to the lower frequency band. The interaction between the end wall and the water caused the resonance peak and the inverse one of the TL curve, which is related to the eigenfrequency of the end chamber wall. The acoustic solid interaction between the expansion cavity wall and the water made the acoustic pressure appear three dimensional high order wave in the lower frequency range, and increased the amount of noise elimination. The interaction between the pipe and the water had little effect on the acoustic performance of the water muffler.