A weighted H_∞ optimization method is proposed to design a robust controller for the flexible system, which solves the problem of unstable characterristics of controller and poor robustness that happens during the design of H_∞ loop shaping method. The idea of phase control is realized by the configuration of the closed-loop poles corresponding to the flexible modes and the closed-loop poles of the high-frequency parts are allowed to differ from the expected poles, which weakens the strict positive real constraints. Combined with the phase control and H_∞ optimization, the desired closed-loop poles can be realized by the controller. The proposed method has the advantage of flexibility, and a clear physical meaning in the design process. The feasibility of the proposed method is verified by two simulation examples.