Abstract The flexible long-span bridge girder is easy to occur Vortex-Induced Vibration(VIV), which is influenced by cross-wind at relative low speed. Although VIV may not be catastrophic, it may be fatiguing to structure and disconcerting to users. The first-order approximate theoretic solution on lock in is deduced by using Scanlan’s nonlinear VIV model and Wilkinson’s correlation function of vortex shedding force to describe vertical and torsional vibration of girder. Moreover, the method is discussed to identify aerodynamic parameters from section model test. Lastly, the spanwise vertical and torsional VIV response of a long-span cable stayed bridge girder are calculated from girder’s section mode VIV test.