Cross-medium vehicles， capable of operating in both aquatic and aerial environments， have significant potential for applications in ocean monitoring， rescue operations， resource exploration， and environmental protection， making them valuable in a wide range of fields. This paper presents the design of a rotary-wing vehicle with foldable arms， codenamed “Feiyi”， which can operate in three distinct modes： underwater cruising， surface navigation， and aerial flight， with the ability to transition between air and water multiple times. In terms of overall design， “Feiyi” utilizes a rotary body structure to minimize hydrodynamic resistance during underwater motion and to reduce storage space when retracted. The vehicle features a propulsion system with four rotors at the front for flight control and four thrusters at the rear for underwater attitude and movement control. To optimize efficiency in both water and air， “Feiyi” adopts a horizontal posture for underwater navigation and a vertical posture for aerial flight， with attitude adjustments during medium transitions managed by a combination of thrusters. A prototype was developed and tested in various operational modes， including underwater navigation， surface cruising， cross-medium transitions， and aerial flight. The experiments demonstrated that “Feiyi” can achieve a flight altitude of 30 m with a height control precision of ± 0.1 m and an angular control precision of ± 0.5°， representing a significant improvement in control accuracy compared to existing vehicles.