The vertical and oblique descend states of Tiltrotor involve extremely dangerous vortex ring state (VRS). To master the aerodynamic load and flow field changes of Tiltrotor in the VRS, the vortex ring boundary mathematical model was established, targeted for corresponding flight simulation and flight test. The CFD method based on slip grid technology was implemented to simulate the VRS of the Tiltrotor, the evolution of the flow field, force and moment of the VRS during the vertical descent flight of the Tiltrotor, with the calculation method of the VRS boundary proposed and the analytical expression of the risk zone of the VRS presented. The results show that the wing of the Tiltrotor reduces the influence of the VRS of single rotor, yet the symmetry of the Tiltrotor rotor increases the risk of unbalanced rolling moments; the Tiltrotor tends to enter the mature VRS when the descent rate vH/v_h is 0.9, with the rotor-lift loss reaching the maximum value of 18%. Given that the descent rate corresponding to the VRS is 0.583≤vH/v_h≤1.516 during the vertical descent state, risk may arise from the overturning caused by the unbalanced lift of the left and right rotors of the Tiltrotor, and from the crash caused by the excessive rotor-lift loss of the Tiltrotor. In view of the dangerous degree of VRS, the vortex ring boundary mathematical models of vertical and oblique descent states are attempted, providing a basis for the design and safe flight of the Tiltrotor.