Abstract: There are frequent gas explosion accidents in urban rain and sewage drainage pipes, which pose a serious threat to people’s lives and property safety. To study the propagation characteristics of gas explosion and the law of gas-liquid two-phase coupling in urban underground drainage pipes, based on the gas-liquid two-phase flow theory and Computational Fluid Dynamics method, a numerical simulation study of the explosion-acceleration-decay process of gas/air mixture under different water depth ratio was revealed. The results show that: when the water depth ratio is less than 0.7, with the increase of the water depth ratio, the long-diameter ratio of the gas phase space increases, the fuel combustion intensifies, and the flame acceleration phenomenon is gradually significant, which leads to the gradual increase of the peak overpressure, the peak overpressure time gradually shortened, and the effect of peak overpressure along the axial direction is more significant. When the water depth ratio reaches 0.7, the propagation of the flame in the pipeline is obviously blocked, and the fluctuation caused by the water shock and the fine water column quickly occupy a small gas phase space, blocking the continuous propagation of the flame, which makes the explosion overpressure appear only near the ignition source. Under the different water depth ratio, in the same zone of the pipeline, the same moment the height of the water being rolled up and the velocity field of the gas phase region is different, and the cryogenic liquid is rolled up to cool and block the high temperature flame in the adjacent zone. Then, due to the macroscopic flow of the gas, the cryogenic gas adjacent to the liquid surface flows to the high-temperature region in the pipeline, resulting in a decrease in the flame temperature in the pipeline. The shock of water and the flying of fine water columns greatly reduce the risk of explosion overpressure. The research results provide a scientific basis for the explosion protection of urban gas lifelines.