This paper points out that the traditional design method of multi blade centrifugal fan are great uncertainty in selecting the structure parameters: lead to many design results, do many tests. Internal flow field of multi blade centrifugal fan was simulated with the computational fluid dynamics software Fluent, and the numerical results are analyzed. Comparison of the numerical results with the test results shows that numerical simulation has good accuracy and reliability. The design program and a design example is gived according to the design theory of multi blade centrifugal fan. Method is verified by numerical simulation for the example. Test results show that the design program is reliable.