Abstract: The design of the safety valve branch pipe of steam pipeline in the nuclear power plant is prone to cause the acoustic resonance phenomenon of the coupling of the flow field and the sound field. Aiming at the acoustic resonance phenomenon caused by the side branch, this paper conducted an experimental study on the effect of the structure of the side branch on the acoustic resonance. The experimental section consists of the main pipe with the diameter of 305 mm and the side branches with the diameters of 64 mm and 44 mm, and the experimental medium is air. The effects of different lengths, diameters and side-by-side structures on the acoustic resonance intensity, characteristic frequency and flow characteristics were studied at the flow rate of 10~65 m/s experimentally. The experimental results show that the second-order hydraulic mode features are significant on the large diameter pipeline, and the St-number of its acoustic resonance peak is about twice the St-number of acoustic resonance peak for first-order hydraulic mode. The increase of the length of the side branch significantly reduces the intensity of the acoustic resonance. The decrease of the diameter of the side branch suppresses the occurrence of the second-order hydraulic mode, changes the flow characteristics and reduces the acoustic resonance intensity. Compared with the single structure, the tandem structure has a significant increase in the acoustic resonance intensity and the flow velocity region of the acoustic resonance with the first-order hydraulic mode and the first-order acoustic mode. Two branch pipes of tandem structure are similar in characteristics.