Based on the boundary condition of field engineering, numerical simulations of 28 conditions of existing diffusers of 3 structure types were investigated by Computational Fluid Dynamics software package, and there were the problems of larger structure resistance and lower diffusing efficiency of these diffusers by analysis of CFD results. The structure outlines of the energy-saving diffuser were constructed by the application of stream function and potential function superimposing. On the basis of numerical simulations of energy-saving diffusers of 5 area-enlarging ratios, structural resistances and diffusing efficiencies of 5 energy-saving diffusers were comparatively analyzed, and the range from 2.00 to 2.31 of the rational area-enlarging ratio of energy-saving diffusers was derived. The optimization area-enlarging ratio of the energy-saving diffuser was presently 2.28 through comparable analysis. From the above, the results show that the coefficient of performance of the energy-saving diffuser is better than 3 existing diffusers.