Energy waste, environmental pollution, loss of human life and property can be caused once underwater gas leakage occurs. It is of great significance to establish an emergency response mechanism for underwater gas leakage and diffusion. A 3D model based on coupled Eulerian-Lagrangian approach is carried out to investigate the underwater release characteristics and diffusion law of gas leakage. The model is validated by the experimental data from literature and the results are accord with the experimental values. Influencing factors such as marine environment, leaking rate, current speed, and water depth are analyzed by numerical simulation. The calculation results indicate that ocean condition mainly affects underwater gas migration behaviors and gas plume consumes the longest rise time under wave and current. Water depth, current speed and leaking rate significantly influence the underwater gas migration but have limited effect on gas diffusion in atmosphere. The investigation will give some advice for emergency response formulation. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.