Abstract: The helicopter vortex ring state boundary is systematically analyzed and studied in this paper. Firstly, the causes of vortex ring state accidents are analyzed, and the physical mechanisms in flight characteristics, rotor performance, rotor inflow, and vortex structure are expounded. The formation of concentrated vortex causes the induced inflow to dominate the vertical inflow on the rotor disk, causing negative damping of rotor thrust and loss of rotor performance, and as a result the instability of subsidence motion. Then, the differences and applicability of various vortex ring state boundaries are compared, the problems of the existing boundary prediction models with strong subjectivity in modeling methods and high dispersion of test data are concluded, and improvement ideas are proposed. Finally, on the basis of the above knowledge, a rotor wind tunnel test to simulate the descending flight was designed and carried out. The test results show that the rotor thrust negative damping, thrust loss and power subsidence phenomenon are presented in the vortex ring state, the rotor thrust loss is up to 30%, and the required power is about 160% of the hovering power when the rotor generates the same thrust as hover; using the negative damping of rotor thrust and the loss of thrust performance, which are concerned in the practice of flight emergency, as the defining index, the discrete points of the critical velocity at vortex ring state boundary are extracted from the test results; in the construction of the vortex ring state boundary model, the strength of the horizontal inflow, the vertical inflow and the induced inflow on the rotor tip vortex is distinguished, and the correction of the momentum theory under different advance ratios and the difference of the rotor tip vortex motion threshold are taken into account. On the basis of the model parameters determined by the least squares method based on the test values, a semi-empirical vortex ring state boundary prediction model is established, and the model is in good agreement with the wind tunnel test results and in line with the trend of flight test results.