In order to explore the energy conservation performance of ocean fishing boat propulsion systems, in this paper, a tuna longline vessel that has realized energy-saving and emission-reducing was taken as an example to analyze the reasons behind, through probing into the structure of its diesel-electric hybrid system, engine load characteristics and the actual using conditions of the vessel. The results showed that the diesel-electric hybrid system of the vessel in use can easily switch between the conventional power and the electricity propulsion systems in accordance with the actual working conditions; by selecting the reasonable working conditions of the main engine, the output efficiency of the diesel engine could thus be improved and the diesel fuel consumption rate decreased, and considering the various working conditions, the system could save energy by more than 15%. The research showed that, at present, using hybrid power technology in tuna longline vessels is feasible and can play a role in energy conservation. With the core technology remaining the same and by adopting some innovative techniques such as multi-machine paddle split system to allow the system greater intelligent deployment role, the hybrid can be further extended to trawlers, purse seiners and other fishing vessels.