To describe the shape and location of free surfaces in the mold filling simulation, parameters including volume filled ratio, surface dimensionless distance, and surface filled ratio for the DFDM (Direct Finite Difference Method) elements were proposed. A model of mold filling process was established, considering the mass, momentum and heat transfer in the vicinity of free surface. It had been applied to an experimental Al casting. In-situ observation and record of actual mold filling process of the casting with special X-ray apparatus were carried out, and were used to validate the simulation results. The validation showed that the model could predict the mold filling process well. It was found that back pressure of gas in the mould cavity had minor effect on change of liquid flow when using pressurized feeding system and sand mould with good air permeability.