Abstract: Wavelet transformation could be used to identify abnormal information in particular locations. Therefore, it has more and more applications in damage detection for simple structures. However, the existence of turning points limits its application in complex structures. In this paper, the wavelet transformation method is used to identify damages in an offshore platform. The subsection technique is used for complex structures, which will introduce more end points and turning points. To combine the wavelet transformation method with the subsection technique, the modal expansion method is introduced in this paper. Numerical results demonstrate that the proposed method can generate good results if appropriate form of modal is used. Introduction of subsection method could guarantee the accurate damage detection for even end points and turning points, so that false detection will be avoided and real damage will not be missed.