<p>This paper investigates the evolution properties of the flow field structure for axial descent rotor based on the unsteady momentum source method combined with the Computational Fluid Dynamics (CFD). The study focuses on analyzing the speed critical value during the onset of or exit from the Vortex Ring State (VRS), and its changing characteristics of the flow field structure. Results from the Lagrangian Coherent Structure (LCS) analysis reveal the switch particularity between the tubular slipstream and the annular vortex, suggesting the criticality change character on flow field structure which occurs in the evolution process of VRS. These criticality features, that is the VRS boundaries described by the flow structure change, as determined by this method, are consistent with the existing theoretical and experimental results, verifying the feasibility of the established analysis method in the study of such boundaries. This work provides a robust framework for studying VRS boundaries and offers insights for further research on the underlying flow mechanism.</p>