Through the simulation of the filling process of LFC（lost foam casting） gray cast iron plate castings,influences of various processing factors on the pressure and thickness in gas gap in lost foam casting process were analyzed by a numerical simulation method.The results show that the filling ability of molten metal is dependent on the speed of the foam decomposition rather than its gravity.The speed is adjusted by automatic change of the pressure in gas gap and gas gap thickness each other.Finally,the mold filling becomes steady-state.The steady-state filling of the molten metal can be realized with the bottom pouring by combining with the theoretical calculation.And the pressure thickness and in gas gap are increased with the increase of the pouring temperature,the pattern density and thickness of coating,and they are decreased with the increase of vacuum rate and coating permeability.