<p>Hybrid aerial underwater vehicles (HAUVs) can operate in water and air and are qualified for complex missions on the air-water interfaces. The unique working conditions put forward higher requirements for the propulsion systems of the vehicles. Present HAUVs’ propulsion systems mostly use propellers, but normal aerial propellers work inefficiently underwater, constraining the underwater applications of HAUVs. Besides, in-depth research of the thrusters, especially during the water-crossing process, is limited. These are two major factors that affect the further development of HAUVs. This paper aims to evaluate the feasibility of the ducted fan propulsion system and design a ducted HAUV with better underwater working capacity. Ducted fans have promising applications on HAUVs for higher underwater efficiency, but the outer ducts can lead to significant thrust loss during the water-crossing process. A novel experimental platform is firstly developed with the capability of collecting dynamic data of the thruster during water- air transition. The new water-crossing strategy and overall design of the HAUV are proposed based on the test results. The ducted HAUV is supposed to accelerate underwater and rush out with a certain speed to overcome thrust loss. A centroid adjustment mechanism has also been designed to realize the underwater motion switch for better efficiency. A prototype named “Nezha-D” is fabricated, and outfield tests are conducted to verify the feasibility of the new design.</p>