Taking the Structural optimum of main metal frame of mast- crane as a sample,this paper discussed the method of Structural optimization of common middle & high flexibility metal tower- frame by dynamic programming.The method of dynamic programming applied to the structural optimum was deducted and the minimum weight design method of tower- frame was practiced The paper gave up the traditional interactive way of choosing type of material,combining with the concrete example,changed the data- table looking up into formula solution,shifting with one by one into directly sieving within selected types,artificially fixing up composed rods into automaticaliy arranging them through the way of maximum twist moment of inertia,thus,accelerate optimization and offered a short way for artificial structural analysis and calculation.