A supercavitation-based thermal desalination method has been put forward, based on which a novel device named Rotational Supercavitating Evaporator (RSCE) has been designed. To investigate the effects of steam extraction on the hydrodynamic characteristics and desalination performance and then determine the ranges of rotational speed and extraction pressure suitable for the operation of RSCE, three-dimensional steady numerical simulations are conducted on the supercavitating flows in RSCE under different rotational speeds and extraction pressures. The results show that steam extraction has little influence on the morphological characteristics and resistance characteristics and is conducive to the generation of the steam in the supercavity. Besides, the steam extraction process enhances the re-entrant jets generated in the region with the radius less than 55 mm under all the rotational speeds other than 3250 r/min and under all the extraction pressures other than 3000 Pa. Despite little change of morphological characteristics, the intensified re-entrant jet increases the length and thickness of the developed supercavity, while decreases the size of the underdeveloped supercavity due to its interaction with the blade. RSCE has low energy consumption when operating under low extraction pressure and the rotational speed of 3750 r/min.