This paper proposes a system of Electric-field Coupled Power Transfer working in the seawater environment. At first, the basic working principle of proposed system is briefly introduced. The electric field coupling is a key part to delivery power in this system, thus this study focus on the calculation of equivalent capacitance of the underwater coupling structure, the distribution of electric field, the influence of transmission distance on it and the power loss. Then, the result is compared with the air coupling structure. Based on this research, there are two important conclusions: (a)The equivalent capacitance of underwater coupling structure is larger than that in the air environment and not affected by the transmission distance; (b)Increasing the operating frequency or the load can reduce the power loss of coupling structure. Finally, the experimental system of electricfield coupled power transfer(ECPT)is designed to verify above conclusions.