Concerned with the Van der Pol type model of vortex-induced aerodynamic loading， fundamental relations among the structural motion amplitude， initial aerodynamic damping， amplitude-related aerodynamic damping are deduced according to the energy balance principle. Further， the basic identification mechanism is obtained in terms of the relation between parameter ε and motion amplitude yT. Based on experimental results of vortex-induced response of a sectional model， which has a typical flat box girder configuration， the relations between the model parameters and the structural motion amplitude are identified. The results indicate that， within the vortex-induced lock-in range，ε decreases almost monotonously as the structural amplitude yT increases. On the contrary， the effective damping induced by the ε-related term increases as yT increases. The structure reaches a steady limit-cycleoscillation state when the three damping components， including the structural，ε-related and initial aerodynamic， neutralize completely. The research of this work shows the initial aerodynamic damping properties determine if vortex-induced resonance is able to be excited and form a lock-in range. Once the relation between the model parameter and the structural amplitude has been identified， the vortex-induced responses of structural damping ratios less than the one used for identification become predictable from the known parameter-amplitude relation.