In order to ensure the stability of trans-medium flight, a switching control method for tandem dual-rotor HAUV is designed. This hybrid aerial-underwater vehicle (HAUV) adopts two different sets of actuators in the air and underwater, and is underactuated during aerial flight, which brings huge challenges to the smooth control of HAUV during water-entry and water-exit. Through the Newton-Euler method, the dynamic model of a new tandem dual-rotor HAUV across different media is established. Considering the switching of actuators in different flight stages, a new type of switching control strategy for the attitude and altitude of HAUV is proposed based on the adaptive super-twisting sliding mode method, and a suitable switching rule is designed to determine the switching timing of controllers. The asymptotic stability of the closed loop system is proved by Lyapunov stability theory. Finally, the proposed control method is tested via simulations of vertical water-entry and water-exit of the tandem dual-rotor HAUV, and the simulation results verify the effectiveness and robustness of the designed controller.