Abstract: This paper presents an interval modal optimization method on uncertain structures. At first, this paper present an optimization with constraint conditions would be transformed into an optimization without constraint conditions by the Lagrange multiplier method, then an interval second Taylor expand function would be build to approximately describe the modal interval of the uncertainty structure with interval parameters. In the interval expression, the second constant term, Hessian matrix, is hardly computed by a common method normally. Therefore, The DFP method would be used to approximatively iterate it. At last, the wanted structural parameters and their interval can be computed by the interval function. During the iteration, the Epsilon method would be used to achieve the result more rapidly and accurately. The optimization method was used in an example of a plate-shell with stiffeners, which prove this method is a useful interval optimization.