Tuna longliner fishing is an important part of the pelagic fishery. The process of tuna fishing consists of  two completely different working conditions: navigation and operation. The speed is high with the condition of navigation, and the speed is low with the condition of operation. In order to meet the requirenents of the two conditions, the electromechanical hybrid power system through the diesel electric independent operation (PTH) mode is used. Electromechanical hybrid power system is composed of two parts, which are the diesel engine propulsion system and electric propulsion system. The high power diesel engine model is used when the speed is high, and the small power electric propulsion mode is used when the speed is low. In the past two years, Shanghai Ocean Fishing Company has built five tuna longliners which are equipped with electromechanical hybrid system. According to the real ship test, fuel saving was 25% and 8% with different types in the hooking stage in the operation period, and fuel saving of voyage reached to 14% in the navigation period. Through calculation and analysis, the initial investment cost of using electromechanical power hybrid system throgh the diesel electric independent operation (PTH) mode could be recovered by way of fuel saving within four years. The fuel saving effects showed that using electric propulsion could save 15% - 20% of the fuel compared with the diesel engine propulsion, which could achieve the effects of good energy-saving, emission reduction and cost reduction.