Abstract: To overcome difficulty of high computational cost in difference sensitivity analysis, efficient derivatives are obtained using approximate-difference strategy based on combined approximations.Accurate sensitivity is obtained under small disturbance verified by numerical examples.Structural volumes are taken as objectives and the response amplitudes are taken as constraints.Shape optimization model of continuum structure under harmonic excitation is established.Some numerical examples are provided and results discussed.Their results are shown to demonstrate the feasibility and validity of the proposed method and model.