Abstract: The approximate full-stress design method associated with the relative difference quotient method, a 2-level optimization algorithm was presented for structural optimum design with discrete variables,which overcame that the imitative full-stress design method could not be used for the structural optimum design with discrete variables,and improved constringency of the relative difference quotient method greatly.The structural optimum design with discrete variables under stress and section size constraints can be directly calculated; and the structural discrete optimization with multi-loading,multi-variable and multi-constraint(including disaplacement constraints)can also be dealt with by this method.A relevant computer program was developed for the method.The method was illustrated by two numerical examples.The results of the examples showed that the algorithm was efficient.