Liquefied petroleum gas belongs to dangerous gas, once the leakage occurs, the consequences are serious. So its security problem is very important. According to the characteristics of LPG leakage, a physical model of leakage and diffusion in the limited space was established, and the diffusion process of the gas leakage was simulated. The internal flow field of the diffusion process was analyzed through simulation results, and the different of the diffusion processes were compared under the different relative humidity. The results showed that due to the moving eddy in the air, the velocity vector of the gas diffusion change from one side was higher than the other side to the other side was higher. The increase of wind speed would accelerate the generation and movement of the eddy, and the concentration distribution of C₃H₈ would change acutely. When the air relative humidity increased, the gas descended faster, and in the plane below the leakage point, the humidity increased, the concentration of C₃H₈ was higher, the alarm time of the liquefied petroleum gas alarm device would be shortened.