Abstract: Because the present algorithm is difficult even unable to solve the problem of structural optimization design of discrete variables, based on the thinking of two-stage optimization, discrete variable and maximum entropy, through introducing additional constraint condition, this problem is equivalently converted to the frame optimization design problem of continuous variables, which can be resolved by carrying out multiple-stage and structure-stage optimization process alternatively. Multiple loading cases and multiple constraints including stress constraints, displacement constraints and dimension constraints are all considered in this paper. Under this condition, the discrete variables frame structures are successfully designed and optimized. Examples show that this algorithm is concise and the iteration is converged rapidly, which can be applied to practical engineering efficiently, especially to the structure optimization of multiple loading cases and multiple constraints.