This study aimed to investigate the three-dimensional flow field simulation of the gas displacement process during modified atmosphere packaging with Pillow-bag, so as to improve the structure of the gas displacement device. One kind of gas replacement new structure of pillow bag packaging machine was designed based on BBFG-120 pillow packing machine; The physical and numerical models of gas replacement device was established which include new structure and the traditional structure; The Gambit software was applied to establish an analysis model, and then the numerical simulation of the gas-flow field was conducted using the Fluent software; The evaluation indexes of the performance of gas displacement device were established and the effect of the pipe′ s position and amount on the properties of gas replacement was investigated. Compared to the traditional structures, the new structure has the following advantages:higher rate of gas replacement, lower consumption of gas, smooth gas flow, and uniform pressure distribution. The new structure′ s performance is better than traditional structures in all aspects.