Abstract: FLUENT software is used as three-dimensional numerical simulation tool to study the law of natural gas diffused in air when a high pressure pipeline was broken in mountainous terrain.The situation of wind speed changeable with the height is taken into account and UDF is programmed into the FLUENT to amend the wind speed.The most dangerous situation of natural gas downward jet is considered and that is compared with the upward jet under different wind speed conditions.Ranges of lower explosion limit concentration and alert concentration under different wind speed conditions for downward jet are given.When natural gas is jetted downwards,diffusion scopes under the condition of the largest concentration boundaries of 1% and 5% at ground of calm wind could reach to 242 m×200 m and 55 m×55 m.However,volume of explosible mass of natural gas is the largest at the wind speed of 5 m/s,which could reached to 45 m×57 m×10 m.