The gas replacement device is the key part of the modified atmosphere packaging machine. The structures of two typical kinds of gas replacement devices with trays were analyzed. The mathematical model of 3-D turbulent gas flow field within the tray was proposed, and then the numerical simulation of the gas-flow field was conducted by means of the Fluent software. The distributions of the flow field within trays for two kinds of gas replacement devices were obtained. The evaluation indexes of the flow field for ensuring the quality of gas replacement within tray were established. Effect of the arrangement mode of gas inlet and outlet on properties of gas replacement was analyzed. The results show that, compared to the typical traditional structure, the new gas replacement devices has better performance in replacement ratio of gas, static pressure distribution, average velocity of flow, and gas consumption. 