Taking a ducted fan rotor as the study object, the reasons why open rotor and ducted rotor produce different lift were researched through the momentum theory. At the same time, the three-dimensional Navier-Stokes equations and SST k-ω turbulence model were used to simulate the complex flow around the open and ducted rotor, which was based on the sliding mesh method. It also analyzed their differences in lift coefficient, power coefficient and efficiency at different rotating speed. The duct restrains tip vortexes and reduces energy wastage markedly, which make the lift coefficient increase 23% and the radio of lift and power coefficient augment 40%. Efficiency is obviously improved and the required power coefficient is slightly increased to 0.05. The results coincide with the theoretical analysis.