Gas mixer is important for modified-atmosphere packaging. There are two kinds of gas mixing equipment, one is based on static principle and the other is dynamic. The dynamic gas-mixing system is more suitable to production and laboratory research. The paper introduces a computer-controlled automatic gas mixer which consists of mass flow meter, proportional control valve and A/D transfer, etc. The unit is based on dynamic flow mixing and not influenced by temperature, which uses mass-flow as control parameter, and it is more precise than others. The test results show that the main parameters are as follows: adjusting range for N₂, O₂ and CO₂: 0～100.00%, mixing precision: (0.381±0.191)% for CO₂ and (0.374±0.165)% for O₂.