Abstract: Taking flood discharge exciting load as driving source and based on the structural dynamic response, this paper puts forward the Modal Coherence Function (MCF) to identify the CMIF curve peak of structure accurately and effectively based on the modal parameter identification method of Frequency Domain Decomposition. The MCF solves the subjective problem of the selection of CMIF curve peak which is the structural frequency or the false frequency (such as noise frequency of flood discharge) and improves the accuracy of modal parameter identification. Then, based on this modal parameter identification method, the dynamic test and structural modal parameter identification of Yingxiuwan hydropower station sluice are carried out on the condition of the flood discharge excitation after the Wenchuang violent earthquake. Simultaneously, the flow-structure couple dynamic characteristic of the sluice is simulated by numeric method with different damage position and damage degree, and the structural damage evaluation sample library of sluice is set up, and then the damage of sluice structure is evaluated. From the comparison between the result obtained from this paper and the conclusion made by the constructor after the field reclamation dam checking, the result is consistent. This method present by this paper provides a new convenient means for detection and evaluation of flood discharge structure after violent earthquake.