The flow field in large threonine fermentor as the research object, by using Fluent software, combining the parameters of threonine fermentation process, and the method of computational fluid dynamics with Multiple Reference Frame and Eulerian multiphase flow model was used to the simulation of single-phase flow and gas-liquid two-phase flow in the mixing process of four stage impeller assembly in fermentor. Distribution of fluid field、power of impeller and volume fraction of gas-phase were also analyzed. The results show that the power of submerged impeller was not affected by the increase of liquid level, and the increase of density in the fermentation process would lead to an increase in the power of the impeller and the two factors are approximately proportional. Moreover the total power of the propeller decreased by 20.9% after aeration, the average volume fraction of gas was stable at about 21.4%.