The full flow field model of a widely used multi-blade centrifugal fan was built, and unstructured grids were used to discrete the computational domain. The moving reference frame is adopted to transfer data between the interfaces of the rotating field and the stationary field. Pressure boundary conditions are specified to the inlet and the outlet. The SIMPLE algorithm in conjunction with the RNG k-ε turbulent model was used to solve the three-dimensional Navier-Stokes equations. The steady and unsteady numerical simulations of the inner flow in the fan at different working conditions were presented using the CFD method. The numerical simulation results were validated by contrasting to the experiment results. The results displayed the characteristics of the velocity field, pressure field, pressure fluctuation at two monitoring points in the centrifugal fan. The results can provide basis for optimizing the fan design and the internal flow, and have important value of engineering applications in the increase of the overall performance in operation.