As a common method for stirring simulation, the value of the dynamic region size of multiple reference frame(MRF) is not accurately defined, and arbitrary value will cause a large error between the calculation result and the actual result, which is easy to cause engineering design errors. In order to explore the effects of different baffle widths and stirring heights on the flow field characteristics in the reactor when the number of baffles is 2 and 4 respectively, the optimal dynamic region size in the PY reactor was determined based on MRF, and the gas mixing characteristics under different dynamic region sizes were simulated. The effects of baffle width and stirring height on gas-liquid mixing in the reactor were simulated based on the optimal dynamic zone size. The results show that the difference of calculation results with different dynamic zone sizes can be as high as 9.7%, and the maximum difference of stirring power is less than 5%. Among them, the diameter and height have a great influence on gas-liquid mixing. The maximum difference of dead zone area with different diameters is as high as 12.32%, while the maximum difference of stirring dead zone with different heights is only 3.38%. When the diameter ratio is 1.4, the simulation error is 1.86%. When the baffle width is the same, the number of baffles is 4, the stirring power is higher and the mixing effect is better than that when the number is 2. The lower the stirring height, the less the gas diffuses to both sides, and the smaller the stirring power. The research results can provide reference for improving the gas-liquid mixing effect in PY reactor.