Energy storage technologies are essential for the mainstream realization of renewable energy. Underwater compressed air energy storage (UWCAES) is developed from mature compressed air energy storage (CAES) technologies and retrofitted to store offshore renewable energy. Existing UWCAES technologies, however, usually operate at off-design conditions when handling fluctuating and intermittent renewable energy, which compromises the round-trip exergy efficiency. To increase efficiency, a multi-level UWCAES system is proposed. The results show that the exergy efficiency of the multi-level UWCAES system varies from 62% to 81% in different working modes. The exergy efficiency tends toward 62% when more energy is stored in the CAES subsystem and approaches 81% when more energy is stored in the design-integrated battery pack.