Abstract: In addition to effective capacity, height is a key parameter to design the fending groin for LNG tankfarm. The liquid pool extension and diffusion behavior in low temperature of semi-underground LNG tank pool are simulated by numerical simulation technique. The impacts of the height of fending groin and the basal area of tank pool on the vapor diffusion velocity at low temperature, the isolation distance of combustible gas clouds and the size of cloud cluster are compared and analyzed. Results have shown that higher fending groin can effectively reduce the volume, size and maximum diffusion distance of combustible gas cloud produced after the LNG leakage while it prolongs the residence time of flammable gas clouds and increases the risk of combustion and suffocation. For the same kind of tank pool, the maximum diffusion distance of the combustible gas cloud is proportional to the basal area of tank pool before the full extension of liquid pool. For the LNG tank farm, it can reduce the safe distance of LNG diffusion by increasing the fending groin height of tank pool.