Unmanned aerial underwater vehicle (UAUV) is a new type of aircraft that can navigate both in air and underwater. A water-air amphibious trans-medium hexacopter was developed based on the analysis of hydrodynamic performance in horizontal and heave motion. First, we introduced the shape, internal structure, and movement principle of the aircraft. Furthermore, based on the assumption of a flat cylinder, a hydrodynamic model of the aircraft was established and its viscous resistance coefficient was estimated. Finally, a series of hydrodynamic analysis was carried out to investigate the rationality of the established hydrodynamic model of UAUV and the relationship between hydrodynamic performance and motion parameters. Simulation results demonstrate that the lift and drag have a parabolic relationship with velocity, hydrodynamic coefficients have nothing to do with velocity, but linearly with the angle of attack, and the acceptable range is 0°-1°.