An approach for detecting the tiny damage of a structure was proposed. Based on the HilbertHuang Transform, the structural damage feature index, i.e., the variation of instantaneous frequency and of instantaneous energy, were extracted, and the dynamic response signals of the composite wingbox excited under a square wave signal were calculated. IMFs of the dynamic response signal were decomposed using EMD method, and the instantaneous frequencies corresponding to IMFs were calculated. The variation of the instantaneous frequencies can be used to judge whether or not the structure is damaged. Hilbert spectrums of the dynamic response signals were obtained using HHT method , and the instantaneous energy corresponding to Hilbert spectrum was calculated, from which the damage information, i.e. the variety quantity of instantaneous energy, can be extracted. And the relationship between the proposed damage feature index and the structural damaged degree was given.