The paper hypothesizes the relations between the density and powder pressure of sand core,builds a model of 2-fluid and 2-pressure,simulates the packing of sand and packed density distribution by DFDM method,analyzes suitability of molding,researches methanism o f sand packing.It is found that it is possible to predict sand packing and the density distribution by compare the theoretical results with the results of experiment in the vacuum suction molding process.