<p>A Computational Fluid Dynamics (CFD) method for variable-diameter (VD) rotor is developed based on the unsteady Reynolds-averaged Navier–Stokes (URANS) equation and moving-embedded grid method to investigate the novel aerodynamic behaviors and the unsteady aerodynamic characteristics of rotor during diameter morphing process, such as spanwise motion of tip vortices, blade–vortex interaction (BVI), and vortex–vortex interaction (VVI). Comparative analysis is conducted to verify the unsteady aerodynamic analysis method in collective pitch ramp change state. The in-depth simulations of three-dimensional (3D) flowfiled during rotor diameter morphing process are carried out in hover and forward flight conditions, and the influences of diameter morphing amplitudes, speeds, and patterns on the unsteady aerodynamic characteristics are revealed. The results indicate that the dynamic change of rotor diameter can induce the aerodynamic inertial behaviors like “hysteresis effects” and “overshoot responses”, accompanied with flow coupling between the spanwise flow and the blade-tip vortices and leading to regional novel BVI and VVI phenomena in the midspan region of the blade. On the whole, the amplitude of diameter variation exhibits a certain linear relationship with the rotor thrust; the unsteadiness of the flow field, such as the overshoot of unsteady load, increases with the VD speed; the diameter change pattern can affect the peak and phase of the aerodynamic loads, and the change degrees of the aerodynamic load are different in the diameter increment and decrease cases. The conclusions can provide engineering guidance for the design of diameter morphing strategies of intelligent VD rotors.</p>