The water-air amphibious unmanned platform has become a hotspot of research because it can move continuously in different media and can perform many special tasks, but the cross-media scheme and structural realisation still face many technical challenges. In this paper, we propose a variable configuration water-air amphibious vehicle based on multi-rotor UAV and underwater glider, which relies on airborne multi-rotor flight, underwater gliding, and fixed-wing double-degree-of-freedom folding variant to achieve high mobility, long endurance, and continuous and reliable cross-media capability. This paper mainly focuses on the hydromechanical characteristics of underwater gliding motion.