Structural damage is too difficult to identification. In this paper, a new method using grey forecasting model to identification the structural damage with past and present data is proposed. Based on the last few years of modal parameters, structural parameters including stiffness and mass of the structural were further predicted the trend using grey forecasting model. Here proposed large structure systems with natural decay and unnatural decay. Natural decay modal parameter can be get from the grey forecasting model, compare with the modal parameter identify currently, by a set value to analysis structural damage. Experimental results verify the theoretical analysis.