Abstract: In the light of double deck floating roof of a 20×10⁴ m³ super-large tank, the numerical calculation scheme ofoverall modeling was proposed and the overall ANSYS finite element model was built. According to the corresponding standards, the stress distributions and sinking displacements under the following four cases were calculated and analyzed, that is, normal floating, two adjacent buoyancy chambers leaking, one edge buoyancy chamber and those around it allleaking, top roof with 250 mm surface water. The results show that the case of top roof with 250 mm surface water is themost dangerous, the maximum stress on float roof is 123.9 MPa in this case and the maximum sinking displacement is 422.4mm. To reduce the whole weight of floating roof, a structure optimization calculation by finite element model was carriedout. The calculation results show that the number of trusses has very little effect on the whole weight of floating roof, buthas obvious increase in support and reduction in stress level, structure optimization should be proceeded from the aspect ofreducing the thickness of steel sheet.