In many practical structures, physical parameters of material and applied loadshave random property. To optimize this kind of structures, an optimum mathematical modelwas built. This model has reliability constraints on dynamic stress and displacement andupper & lower limits of the design variables. The numerical characteristic of dynamicresponse and sensitivity of dynamic response based on probability of structure were deducedrespectively. By equivalent disposing, the reliability constraints were changed intoconventional forms. The SUMT method was used in the optimization process. Two examplesillustrate the correctness and practicability of the optimum model and solving approach.