Abstract: A reduced order model and the corresponding system identification method are presented for constructing nonlinear structural model in aerodynamic system. And for the first time it is applied to constructing nonlinear structural model in wind-induced vibration of long-span roof. Reduced order model is adopted for the nonlinear structure and it is established by system identification of a high fidelity finite element model. Strain energy functional of the system is expressed as a polynomial of the structure’s modal amplitudes in modal coordinates. And the unknown coefficients of the polynomial are determined by modes and strain energy information calculated using a high fidelity finite element model. Wind-induced responses of a long-span roof are calculated using the proposed method. The results agree well with those obtained from the high fidelity finite element model and experiments. The current reduced order model is suitable for constructing nonlinear structural model.