This paper proposed an identifying method of scour condition of cable-stayed bridge pylon by tracing the dynamic index. Firstly the quantitative relationship between the identification index and scour depth is theoretically simulated by a series of parametric study. Once the identification index is obtained based on the monitored dynamic performances during the bridge routine measurement,the scour depth of pylons at the moment of monitoring can be directly deduced by the pre-obtained quantitative relationship. In order to investigate the feasibility of this method,two identification indexes such as the natural frequency of bridge and deformation of modal flexibility were proposed. As a case study,the relationships between these two identification indexes and scour depth of pylon were carefully analyzed based on Finite Element (FE) model of Ningbo Zhaobaoshan bridge (a cable-stayed bridge). The results indicate that the pylon scour depth of cable-stayed bridges can be quantitatively identified by tracing the identification indexes. Especially,the indexes including the deformation of modal flexibility built by the vibration modes of vertical bending of girder and transverse bending of pylon show much more sensitive and better identification effects than those of other indexes. It can be concluded that the proposed identifying method for pylon scour depth by tracing the dynamic index has the advantages of accurate calculation logic,convenience,and good economical efficiency without underwater operation. The scour depth can be correctly predicted as long as the dynamic performances of bridges are accurately measured and traced.