For the cross-sea optical communication system, the power efficiency problem caused by the power supply structure between shores is the main factor that limits its capacity. Multiple single-mode fiber (M-SMF) multiplexing is the main solution to improve the capacity of communication submarine cables at present. However, the maximum deployed fiber count in a submarine cable is usually limited up to 32 due to the considerations of cable mechanics and deployment engineering. As a consequence, high-density spatial division multiplexing is expected to show its advantages in the field of submarine cable communication. The power efficiency formula based on multi-core fiber (MCF) submarine cable is deduced theoretically, the power efficiency characteristics of MCF submarine cable and M-SMF submarine cable are compared, and the effects of marginal parameters such as insertion loss of multi-core coupler and inter-core crosstalk on the overall power efficiency of the system are analyzed. The results show that the optimal number of trans-Atlantic 4-core fiber and trans-Pacific 4-core fiber is 86 and 14 respectively, and the optimal number of trans-Atlantic 7-core fiber and trans-Pacific 7-core fiber is 50 and 8 respectively. When fiber count is limited to 32, the power efficiency of 4-core fiber can be improved by 2.50 times and 1.13 times compared with M-SMF submarine cable in trans-Atlantic and trans-Pacific scenarios, and that of 7-core fiber can be improved by 3.20 times and 1.13 times compared with M-SMF submarine cable in trans-Atlantic and trans-Pacific scenarios.