Abstract: An analytical method for predicting rotor vortex wake in low speed and hovering flight is described to investigate the motion of the helical tip vortex. Beginning with the prescribed wake model, a Semi-Empirical Correction for the vortex core effect on rotor wake is made and free wake calculation is carried out. Then on the condition of circulation and wake geometry convergence, the rotor thrust is calculated to satisfy convergence criterion of thrust. Finally, as an example of its engineering application, the calculated downwash velocity field along the rocket launch line is presented and simply analysed. In terms of theory, the calculating method developed here may provide referable basis for further studying the formation mode of the tip vortex and vortex core interior structure. Also the method is practical for predicting rotor induced flow field in engineering applications.