To initiate the experimental study of natural gas diffusion in sewage pipeline and evaluate the sewage pipeline risk, a new outdoor full-scale water cycling experimental pipeline system was built. The concentration distributions and characteristic time were measured in a comprehensive monitoring system. As the rate of the gas leakage flow decreases, the alarm time increases significantly. It is found that the influence of water flow direction on methane gas diffusion is different under varying gas flow rates. Furthermore, the methane diffusion rate in sewage pipeline is also a function of water flow rate and water level height. Based on the experimental results, the corresponding suggestions for prevention and control of gas leakage accidents in sewage pipelines were put forward to improve the urban public security management system and the security capacity of urban public security. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.