Abstract: By studying trial calculation problem in design calculation for lower node bending moment and corner of large-sized oil tank, it is considered that trial calculation solution is much closer to actual work situation than approximate solution but trial calculation solution is not convenient for application in actual design work, with tedious calculation and liable to make mistakes. It points out that the kernel of trial calcu- lation method is to find a solution that can satisfy L values of both equations. Based on the kernel problem, it points out a new solution using optimum theory to solve both equations and establishes a mathematics model for the new solution. Explains executive procedure of the new solution through actually calculated case and verifies the feasibility, science and practice of the new solution.