Abstract: To address the challenge of high-torque operations encountered during the replacement of chemical injection metering valve (CIMV) in a subsea production system in South China Sea, this paper presents the design of a ROV-compatible subsea torque-amplifying tool. Utilizing a planetary gear transmission mechanism, this tool amplifies the input torque by a factor of 2.7. It is equipped with a torque limited low torque tool (TLLTT) to precisely restrict the output torque within a reasonable range. The tool features a concise structural design, facilitating easy operation by ROV manipulators. By optimizing the ROV operational procedures, the subsea replacement of the high-torque CIMV was successfully completed. Additionally, an innovative approach involving a subsea basket was employed to transport the CIMV to the seabed, significantly reducing operational risks while substantially improving operational efficiency. This torque-amplifying tool and its associated operational methodology provide an innovative technical solution for CIMV replacement, and effectively overcome the insufficient torque capacity of ROV manipulators. It offers valuable technical reference for valve installation and removal in similar subsea facilities, and demonstrate significant engineering application value. Furthermore, it provides important guidance for the future safe operation and maintenance of subsea production systems.