Abstract: The model reduction is the precondition for the model updating based on the frequency responses. It is only with an accurate model reduction method, that the proper model updating can be ensured. With the view to accurately reflect the dynamic characteristics of the model for the spacecraft structure in the medium-to-high frequency band, an improved SEREP model reduction method is proposed in this paper, based on the modal analysis of the structure, and the increase of the dimension of the linear subspace corresponding to the model reduction by adding extended master degrees of freedom. It is shown by a spacecraft structure example that the precisions of the eigenvalues and the frequency responses are increased with the improved SEREP model reduction, thus the reduced model can accurately reflect the dynamic characteristics of the structure.