With the increase of LNG power ship, the structure form of the fuel tank has been paid more attention. In this paper the advantages and disadvantages of LNG powered ship fuel tank with double shell tank structure or independent C type single shell tank structure are discussed from the initial evaporation rate control and the construction cost. According to the characteristics of the fuel tank for LNG power ship, the weight of the steel structure for the double shell fuel tank and the single shell fuel tank is calculated based on the design of four different kinds of fuel tanks, and the evaporation rate is simulated and predicted by use of ANSYS finite element software. Through the comparative analysis, it is found that the two kinds of structure can both meet the requirements of the specification. Under the condition of the same volume, shape and thickness of the insulation layer, the weight of the double shell tank structure is about two times as that of the single shell tank structure, while the evaporation rate about a quarter.