Abstract: Discusses the problem of tentative calculation in oil tank's design with variable wall point design method. It is thought that a key problem in the tentative calculation is to solve an equation that is hard to get an accurate solution. In light of this problem, it point out the shortcoming of the tentative calculation method in engineering application and puts forward a new thinking and solution to solve tank wall thickness with general optimizing calculation method, and gives a mathematics model for the new solution and shows an arithmetic example to describe practical application of the new solution.