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• A system of hydrogen-air fuel cell and battery (FCBS) is proposed for underwater. • The coupling design issue of FCBS and the underwater vehicle is studied. • FCBS is superior to single hydrogen- oxygen fuel cell system in endurance. • A 65-min underwater experiment of FCBS is completed. At present, most underwater fuel cell power systems are fed up with pure hydrogen and pure oxygen (FC), which suffers from high capital costs and potential fire hazards. However, all commercial fuel cells are designed in an open-air environment, so it's not available directly. To enhance the range and reduce costs of unmanned underwater vehicles, this paper proposes an underwater energy system of the hydrogen-air fuel cell and battery (FCBS), solving the problems of artificial air intake, exhaust gas treatment and energy management strategy. Employing cruise time, range and mass density as evaluation indexes, the coupling design between FCBS and the key parameters of an unmanned underwater vehicle including the length and velocity, are analyzed contrastively and optimized. The results shows that as the length expands, FCBS can be boosted by over 40 % in cruise time compared to FC, but this advantage fails in very high velocity. Moreover, the optimal velocity for the maximum range of FCBS is 1.4 m/s, and the optimal velocity of FC is 2.0 m/s. Finally, the hybrid system scheme of FCBS is verified by the underwater closed cabin experiments. The results prove the operating feasibility and dependability of the underwater hybrid system with a hydrogen-air fuel cell. [ABSTRACT FROM AUTHOR]