Abstract: Analysis of urban gas pipeline's dynamic leakage characteristics can be made to effectively predict the harms and reach of leak gas after diffusion. This paper proposes a leakage calculation model after analyzing the dynamic leakage process and state of urban gas pipeline. According to the changes of dynamic leakage rate, the Green's function is applied into the fluid diffusion differential equation to obtain a synthetic model of dynamic diffusion. Combined with mathematical simulation for actual cases, it is recognized how and where a leak gas may diffuse under different conditions and states. The results show that leakage ratio, wind speed and atmospheric stability affect urban gas pipeline's leakage and diffusion. Higher wind speed or leakage ratio will further spread the diffusion of leak gas. When the wind speed is greater than 3 m/s, the downwind diffusion will decrease, the harms of jet fire will increase, while the harms of vapor cloud explosion will decrease, as the wind speed increases. When the wind speed is lower, vapor cloud explosion will bring more extensive harms than jet fire, but when a serious leakage occurs at a high wind speed, jet fire will be more harmful than vapor cloud explosion.Therefore, emergency evacuation distance shall be defined depending on circumstances.