The work aims to improve the structure of the gas replacement device in DF-520 MAP machine and conduct internal flow field simulation to solve the problems of low accuracy and replacement rate and disordered internal flow field in operation of existing gas replacement device. The FLUENT software was used to make numerical simulation and comparative analysis for old and new gas replacement devices with boundary conditions of 20 kPa inlet pressure and gas mixture ratio of 2% O2, 28% CO2 and 70% N2. Subsequently, the accuracy and replacement rate of two models were compared via prototyping test. Numerical simulation results indicated that compared with old configuration, the replacement chamber of new configuration had relatively even gas flow rate, smaller strong current and vortex area, high-consistency gas flow direction and rapid gas replacement speed. Prototype test results showed that the average gas replacement rates of new and old configurations were 99.2% and 95.6%, respectively. Meanwhile, for new configuration, the standard deviation, average relative deviation and coefficient of target gas concentration ration were smaller than those of the old configuration. The optimized gas replacement device has more stable gas flow distribution and higher efficiency, with replacement accuracy meeting the design requirements and it is suitable for packaging processing and can provide a theoretical reference for further research.