Gas leakage is one of the most common failure modes in high-pressure duct of civil aircraft. This paper conducts a numerical simulation of gas leakage from a high-pressure duct. The calculation results show that the leakage scenario belongs to a free jet with shock wave. The flow development characteristics are essentially the same as typical free jet, but the initial segment of the jet is longer, and the velocity and temperature at the front part of the basic segment are higher. With the full development of the flow, the velocity and temperature on the axis of symmetry gradually approach the theoretical values calculated by the typical free jet. The simulation method can accurately predict the flow and thermal diffusion characteristics of gas leakage in high-pressure duct. When designing the numerical simulation scheme, special attention should be paid to the settings of the external flow field length, external flow field environmental temperature, and the temperature and pressure of the fluid inside the duct.