A novel desalination device named rotational supercavitating evaporator (RSCE) has been proposed and designed by utilizing supercavitation effect. With special focus on the spatiotemporal evolution of rotational natural cavitation, the hydrodynamic characteristics of cavitating flows in RSCE under different rotational speeds are studied by the visualization experiments and three-dimensional steady numerical simulations. The results of the visualization experiments show that with increasing rotational speed, the cavity morphology develops from several transient isolated bubbles moving with the blades, to blurred partial cavity, and finally to transparent supercavity with nearly constant size. Numerical simulation can predict the development of the cavity morphology in the experiment qualitatively and quantitatively. Vapor phase structures are shed at the tail of the cavity due to the reentrant jet, which are in the forms of single smaller bubbles and U-shaped vapor phase structures under lower rotational speeds and of cavitation clouds and cavitating filaments containing strings of bubbles under higher rotational speeds. Vortex structure is captured based on Q-criterion and encloses the cavity in the radial direction, wherein the periphery of the cavity is enclosed by a single tip vortex tube which can explain the generation of drifting stripe-shaped cavity under higher rotational speeds due to tip vortex, and the cavity tail is enclosed by two vortex tubes split from the single tip vortex tube. A power-law empirical formula for the dimensionless supercavity length versus the cavitation number considering the effect of rotation is obtained by fitting the experimental data on fully developed supercavitation.