Abstract: The structural system identification with incomplete input and output information is studied.The substructural dynamic equations and parameter identification equations are established,which can be applied to various structures.According to the property of input information in substructures,a decomposition algorithm or a statistical average algorithm is used.The two algorithms identify the structural parameters effectively even when the excitations of part degrees of freedom or seismic excitations are unknown.Combined with decomposition algorithm or statistical average algorithm,the substructure method proposed in this paper offers an efficient approach for the dynamic detection of engineering structures with limited observations.For verification purpose,both the noise-free and noise-included output responses are considered in the numerical examples.Results show that the structural parameters and loads can be identified robustly in all cases.