Helicopter vertical descend state contains extremely dangerous vortex ring state. In order to improve the understanding of the vortex ring state, to master its aerodynamic load and flow field changes, and to provide experimental data for the corresponding numerical simulation and flight simulation, the aerodynamic load and flow field measurement test of Bo-105 rotor model under vertical descend state was conducted in Φ5 m vertical wind tunnel using helicopter vertical flight test rig and the large filed PIV measurement system. In the case of similar tip Mach number, the average thrust, average power, and wide range PIV flow field images of this rotor model under different collective pitch and descent speeds were obtained, the rotor model aerodynamic load variation was analyzed, and the flow field development process under different descent speeds was captured. The test research shows that the thrust and power decreased sharply after the vertical velocity of the rotor model dropped to 8 m/s, and the root-mean square error increased significantly. The smaller the collective pitch, the easier it was to enter vortex ring state, and the faster for the rotor model thrust to decrease with the increase of vertical descent speed. The main cause of instability of blade tip vortex structure is the vortex-pair structure. When the distance between adjacent blade tip vortices was smaller, the blade tip vortex was more unstable. The formation of vortex ring by the blade tip vortices gathering together and the dynamic evolution of the vortex ring structure were the key factors in the sudden change of rotor performance.