To investigate the damage and deformation mechanism of large scale steel floating roof oil tank under the combustible gas explosion, according to the Von Mises yield condition and strength theory, the generalized yield function and failure criterion of tank wall under blast loading is established. A finite element model of 15×10⁴ m³ floating roof tanks has been established by explicit package ANSYS／LS-DYNA, and the dynamic responding processes such as displacements, acceleration, stress and strain of the tank walls structures under blast loading have been simulated. The results show that the failure mode of the floating roof tank is collapse and buckling on top of the impact surface tank walls. The yield range first appears at the stagnation area and then propagates to the neighboring parts, and the irregular plastic hinge circle obviously appears around the deformation area, which results in the concaved bucking of the tank inner surface. During the whole process, the inner liquid not only impacts on the structures, but also absorbs and consumes part of the blast energy.