Abstract: Cable-stayed bridge(CSB)is an important structure on traffic lines.Monitoring the bridge health is very necessary.In this paper basic steps of structural dynamic damage detection are discussed and a three-step method for structure damage diagnosis(TSMSDD)is proposed.The method is verified using the Yamen cable-stayed bridge of Xinhui city in Guangdong Province.The result shows that the proposed TSMSDD is effective for damage detection of long span cable-stayed bridge.