Abstract: his paper mainly studies the H∞ identification of Flexible Space Structures(FSS).The results denote that the low dimensional model oriented to control can be obtained by combining the estimation of the unknown parameters and truncation of the infinite dimension of the FSS based on input output data.As the dimension of the model is determined,the error between the model and the system may be minimum.As the dimension of the model increases, the error may be arbitrarily sm 11.This is the basic idea of H∞ identification of FSS. To do so, with the satellite having flexible arm as background this paper firstly analyses the dynamics properties,then introduces the concept of Had identification of FSS, thus constructs the H∞ identification algorithm of FSS, and proves the convergence of the algorithm.