In summer, the mixing flow fans (MMF) are usually used to reduce the heat stress in dairy cows. In order to improve the cooling effect of mixing flow fans (MMF) in summer open dairy house, numerical simulation and field experiment were adopted to optimize the performance of MMF, without changing the dimensions and blade shape. Totally 24 geometry models of MMF with diffusers of different angles (90°, 120°, 150° and 180°) and different lengths (150mm, 250mm, 350mm, 450mm, 550mm and 650mm) were established for CFD simulation, and the angles and lengths of diffuser were optimized by taking the fan performance, wind speed distribution in the distance of 0.5~1.0m from MMF axial and nonuniform coefficients as evaluation indexes. In this thesis, the diffuser angle 90° was taken as an example, the best diffuser length of the diffuser angle 90° was elected according to the fan rotation speed distribution and the diffuser effect of overall calculated filed. The analysis of other diffusers angles(120°, 150° and 180°) used the same methods. The angle and length of the diffusers (90°/450mm，120°/350mm，150°/250mm) were better to improve the MMF air flow and flow field uniformity, making use of analysis the jet length and average axial velocity. Finally, according to the simulation and experimental results, the 150°/250mm diffuser increased the wind quantity and efficiency ratio by 3.80% and 11% respectively, and led to greater axial velocity and better nonuniform coefficients. This improvement provides a theoretical basis for facility agriculture equipment while reducing production costs.