SummaryThe existing wireless power transfer systems for autonomous underwater vehicles (AUVs) have problems due to long transmission distances and the unstable attitudes of AUVs, which result in poor system performance. To solve this problem, this paper proposes a lightweight universal variable ring‐shaped magnetic coupler for the shape characteristics of AUVs and the charging platform environment. In particular, the size of the magnetic coupler transmitter can be varied according to the size of the AUV, resulting in a close fit between the magnetic coupler transmitter and receiver, reducing the transmission distance and fixing the AUV attitude. To achieve a lightweight receiver and reduce the burden on the AUV, the design is optimized in terms of both the system topology and the magnetic coupler structure. First, the LCC‐S topology is used to reduce the number of components on the secondary side. Second, the use of a nanocrystalline ribbon as the core material effectively reduces the weight of the magnetic coupler receiver. Finally, a 300‐W experimental platform is built to verify the functionality of the designed magnetic coupler. The experimental results show that the system has a DC‐DC transfer efficiency of 86%. With the variable ring‐shaped magnetic coupler, the charging power can be increased by 7.2% when the transmission efficiency remains unchanged.