The image vortex model is widely used in lifting line method to predict the flow induced by the shroud and hub surface in ducted fan, but it does not perform well in lift fan design due to the zero far field inflow. In this paper the theoretical defects and numerical error propagation characteristic of the image vortex model in simulating duct surface are studied. Then the vortex rings correction based on image vortex model is verified and applied in lifting line method. In order to test the improved method, a 150 mm diameter lift fan is designed by the modified lifting line method, after that, analyzed by the modified lifting line itself and RANS solver respectively. The results show that the contribution of the vortex rings at the rotor accounts for 50% of the total induced velocity, and it accounts for 35% at the stator, which verifies the necessity of the vortex rings correction. The performance of the rotor and stator geometry is consistent with the design expectation, the efficiency is up to 90.97% by numerical simulation and 88.32% by experimental measurement, which confirms the reliability of the modified lifting line method developed in this paper especially on lift fan design.