Abstract: The boil off gas (BOG) processing system is one of the key issues directly relating to the energy consumption, safe and steady operation of LNG repository. In order to reduce huge losses caused by the venting of BOG, it is necessary to adopt the BOG recovery technology in LNG repository. To reduce the energy loss caused by the heat leakage, three BOG reliquefaction processes (methane refrigerant liquefaction cycle, the mixed refrigerant liquefaction cycle and nitrogen expansion refrigeration liquefaction cycle) were proposed based on analyzing the BOG recovery methods of LNG repository. Using Aspen HYSYS software to simulate the BOG reliquefaction process, the refrigerant flow and the power consumption of the system needed in three processes were compared and analyzed, and the most suitable BOG reliquefaction process (the mixed refrigerant liquefaction cycle) in LNG repository was determined with the effective energy related theory. The shaft power of compressor is 15.30 kW less than that of the methane refrigerant liquefaction cycle process and 146.42 kW less than that of the nitrogen expansion refrigeration cycle process. Moreover, the effective energy loss of mixed refrigerant cycle system reduced 22.06% compared with that of the nitrogen expansion refrigeration cycle process, and lowered 35.78% than methane refrigerant liquefaction cycle process; the required refrigerant flow was small. It is applicable to LNG repository with large storage scale.