Duct fans play an important role in WIG propulsion system.The vortex system of duct fan can be divided into two parts

inner sheet and boundary sheet

which are induced by each other.It is assumed that the boundary sheet should eliminate the radial velocity disturbance of inner sheet

match the discontinuity between the intersections of boundary sheet

not induce the inner sheet

and ensure the constraint that the sum of vortices equals zero.Based on the classical theory of propeller

Kutta conditions

vortices induced equations

and assumptions of duct fan

a numerical model for calculating duct vortex intensity of fan blades is set up in this paper.Results show that the vortex intensity reaches its maximum value at the blade tip for the duct fan.This intensity profile is much different from that of the general propeller.