During the process of designing motors with radial vent ducts, the influence of radial vent ducts on the loss of the iron core length should be considered when calculating the effective length of the iron core. Although there are mature mathematical methods for asynchronous motors, there is no mature mathematical method to calculate the loss width of vent ducts for permanent magnet synchronous generator (PMSG). Combined with the 2D magnetostatic solver of ANSYS Maxwell software, a method for calculating the flux density distribution of motor along the axial direction of gap is proposed. The effective iron core length of PMSG with radial vent duct is obtained, and the simulation data are highly consistent with the experimental data. The effectiveness of this method is proved.