Jet fire may cause damage to people and facilities, which will lead to the escalation of accident. Jet fire caused by natural gas pipeline leakage could cause not only the waste of resources, but also the environmental pollution and serious damage to life and property. In order to study the hazard of jet fire in long distance natural gas pipeline with high pressure, CFD simulation based on gas combustion science and fluid mechanics mechanism was carried out to investigate the influence of leakage diameter, leakage pressure and ignition delay time on the length of flame and the damage range of thermal radiation. The results showed that at the initial stage of jet fire , the length of jet flame increases linearly with the combustion process under the action of initial ignition energy. There will be an unstable state for a period of time due to heat diffusion and insufficient supply, jet fire reaches an equilibrium state until the heat generated by the diffusion and the combustion of the jet fire reaches an equilibrium state. The front of the gas pipeline jet fire forms a spherical flame firstly, and then develop into a stable flame gradually. The length of jet flame has experienced three stages of straight rise, dynamic fluctuation and stability. The flame length increases with the increasing of leakage diameter and leakage pressure, and the increase amplitude decreases gradually. The flame profile becomes more obviously with the increase of leakage pressure. The damage range of thermal radiation level increases with the increase of leakage aperture and leakage pressure, and the influence of leakage aperture is higher than that of leakage pressure, and the growth of light injury range is the fastest. The ranges of minor injury and death at 10 MPa were 1.55 times and 1.72 times of those at 2 MPa. When the leakage diameter is 100 mm, the ranges of minor injury and death are 2.03 times and 3.08 times of those at 20 mm, respectively. At the initial stage of ignition, flame length and damage range increase significantly with the increasing of ignition delay time. When the combustion reaches stability, ignition delay time has little influence on flame length and damage range.