To investigate the characteristics of the flow field of high solids in solid state fermentation reactor with mechanical agitators and optimize the design parameters of mechanical agitators, computational fluid dynamics (CFD) method was used to simulate the flow field of five types of mechanical agitators, and parameters such as ratio of impeller diameter and stirring speed were analyzed quantitatively through the indexes of stirring power, mixing time, mixing energy, and the percentage of dead zone. Simulation results show that the high solids had a small range of disturbance and a large percentage of dead zone, indicating that it cannot be effectively mixed through double blade and turbine agitator. The high solids had the largest range of disturbance, the smallest percentage of dead zone, and the biggest stirring power through double helical agitator. For the impeller diameter of the double helical agitator, the dead zone decreased with the increase of the ratio of impeller diameter, and when the ratio of impeller diameter increased to 0.75, it no longer decreased, while the stirring power increased with the increase of the ratio of impeller diameter all along. With regard to the stirring speed of the double helical agitator, the stirring power increased linearly with the increase of the stirring speed, the mixing time decreased along a parabolic curve, and the mixing energy gradually increased. Considering a balance of mixing performance and mixing power, the double helical agitator with a stirring speed of 20 r/min and the ratio of impeller diameter of 0.75 was the best agitator design in this reactor.