Abstract: In order to catch the figure of internal flow field of multi-stage centrifugal fan, centrifugal fan of a pneumatic grain sucker was used as the research object in this thesis, three-dimensional steady and compressible computation was performed for its whole internal flow by using FLUENT 6.3 software. During the course of the calculation, the accuracy and efficiency of computation were overall considered，the model of all channels were built by using Pro/E as a solution domain, the MRF（moving reference frame）method was used to realize real-time transmission of flow parameters among each stage impeller, the RNG(Renormalization Group) k-ε turbulent model and method of Roe-FDS（Roe flux difference splitting）flux difference with first order upwind spatial discrete schemes were used to realize the simulation. According to the result, the distribution diagrams of pressure and velocity in the main area were provided, through analyzing it was found that flow velocity of gas between each impeller maintains the same level, but static pressure and total pressure increase gradually, and except the third impeller, the distribution of flow field in the first and second impeller is mostly symmetrical. Through analyzing the internal flow field of volute, the design defect of this fan that gas could not flow smoothly in the small part of volute was discovered, and the advice for improving volute structure was given, which provides reference for improving the efficiency of the fan.