The embodiment of the invention discloses an on-line underwater oil-gas leakage safety monitoring device and method. The device comprises at least two underwater sonar probes arranged on an oil-gas production line of underwater production equipment at intervals, a camera arranged on each underwater sonar probe, a controller connected to each underwater sonar probe and the corresponding camera, anda maintenance device connected to each controller, the controller is used for determining the leakage position according to the underwater acoustic signal sent by each underwater sonar probe and theimages sent by each camera and transmitting the target action route from the current position of the maintenance device to the leakage position to the maintenance device, and the maintenance device isused for being moved to the leakage position for conducting the maintenance operation of the underwater production equipment according to the target action route. By means of the on-line underwater oil-gas leakage safety monitoring device, the real-time monitoring and maintenance of the underwater oil and gas leakage are achieved, the safety of the oil and gas production is ensured, and the pollution of the marine environment is avoided.