The iteration synthesis of substructures of the structural system in subspace is described and the iteration of the characteristic equation of the system in the subspace is derived. Therefore，a number of the low-order modes of the large structure can be found. According to this method only the mass matrix and stiffness matrix of every substructure of the structure must be stored in the computer. According to the preselective accuracy it is possible to solve the problem of the eigenvalue of the large structure simply and effectively.