Aiming at the problem in dynamic leakage of urban overhead gas pipeline,considering the impact of different pressure levels on leakage and diffusion, high pressure (2.0 MPa), sub high pressure (1.0 MPa) and medium pressure (0.4 MPa) three kinds of pressure grade pipe to simulate were selected.Firstly, the leakage rate of the critical flow and the sub critical flow were calculated by using the leakage rate calculation mode.Secondly, the leakage rule of different pressure pipelines was obtained.The Fluent software was used to simulate the unsteady state leakage, and the risk range of natural gas diffusion was obtained.The results showed that if the pressure in the tube was greater,the quality of residual gas in pipe and the time of leaking was higher and the risk range of natural gas was lager when the volume of pipe and the diameter of leak were fixed.With the continuous leakage of non steady state, the leakage rate was getting smaller and smaller, the risk of natural gas was also getting smaller and smaller.