Abstract: Gas replacement device is one of the key parts for modified atmosphere packaging (MAP) machine. It is meaningful for investigation and optimization design of the gas replacement device by computer to simulate the gas flow field under different gas-exhausting types and gas-charging pressures. A physical model of MAP with trays was established to analyze gas replacement process depending on the gas replacement mechanism. The RNG k-ε turbulence mode was used to establish the three-dimension turbulent gas flow field during the gas exchange process. With the equivalent area, effect of the gas-exhausting type on gas replacement properties was analyzed, the optimum layout of exhausting mode was determined, and then an appropriate working range of gas-charging pressure was acquired. For three gas-exhausting types, the difference of gas replacement properties was no notable within the vacuum degree range of 0.04-0.1 MPa. Along with the decrease of vacuum degree, the gas-exhausting type of centre hole possessed the best properties. For the exhausting type of centre hole and gas-charging of four holes around the centre hole, the high precision and efficiency of gas replacement could be obtained above 0.5 MPa of the gas-charging pressure.