This paper proposes a bidirectional undersea capacitive wireless power transfer system (BD-UCWPT) for the autonomous underwater vehicles (AUVs). The two H-Bridge circuits are adapted in the proposed BD-UCWPT system to achieve the bidirectional power flow. The H-Bridge circuit is built based on the GaN devices. As a result, the high switching frequency, and low power loss could be reached. The two LCLC compensation networks are respectively used in the transmitter side and receiver side. The simulation work is conducted with the Ansys Maxwell, PSIM and MATLAB. In the experiments, two seawater bags are added in the middle of two metal plates of C S1 and C S2 to model the seawater condition, respectively. In order to verify the theory analysis, a 100W experimental BD-UCWPT system is constructed. The experiments have been conducted in air and in seawater, respectively. The simulated and experimental results are compared with each other. It shows that, with this BD-UCWPT system, the power of AUVs could flow bidirectionally. The stable operation and high efficiency of BD-UCWPT system is verified. The research work in this paper could facilitate the application of wireless power transfer (WPT) system for AUVs and other undersea devices. INDEX TERMS Wireless power transfer system, bidirectional, undersea, autonomous underwater vehicles (AUVs).