Centrifugal fan design conditions flow less vortex and separation, complex internal structure. The general software and numerical method can easily realize the performance prediction of fan, the vortex and separation flow are more in non-design condition, the flow efficiency is low, and the numerical simulation is not easy to realize the performance of fan. In addition, due to the limitation of cfd software and calculation technology, it is mentioned here that the flow field calculation of the whole machine can only realize the coupling numerical simulation of impeller and volute, but also can not realize the whole machine calculation in the real sense. The three-dimensional flow field calculation of impeller-volute-inlet-connected pipe. The centrifugal fan is based on the improved engineering method of pneumatic design by using pneumatic design software and design experience. Based on the selection of some empirical coefficients, the aerodynamic design diagram of the fan with good performance is given, and then the three-dimensional turbulent flow field inside the fan is calculated by cfd software, and the full pressure and efficiency under a given flow rate are obtained. Because this can estimate the fan performance in the design stage, this is the past engineering design method can not get if the prediction performance is satisfactory, then change the original experience design parameters, re-carry out the fan engineering design, get a new fan aerodynamic diagram, Then 3D flow field calculation is carried out to obtain the prediction performance of the new fan. If not satisfied, then re-carry out the engineering design fresh air machine, then calculate the estimated performance, and then do the prototype test, until the engineering design fan estimated performance and measured performance similar and satisfactory prototype, the design is completed, The whole design process is called the modern design method of centrifugal fan.