The assessment of the safety and reliability of double hull structure under the excessive lateral pressure requires the confirmation of buckling of the skin plate panels and the post-buckling behavior of the global structure. Recent progress of the analysis tool and expanded capacity of the computer system enables to solve such complicated problems gradually. However these analysis have not been effectively applied in the actual design stage yet because the global hull structure model is too large for this kind of nonlinear analysis. In order to accurately evaluate the post buckling behavior of whole double hull structure in a practical time, the authors have developed the new assessment system by the linear FEM analysis introducing simplified plate element with equivalent stiffness of buckled panel. This system enabled the evaluation of the post buckling strength of whole hull structure in very short time, and thus, the design for optimizing the arrangement and reinforcement of structure.