Abstract: In this paper,a free-interface mode synthesis method for vibration system of contains stiffness coupling component is described.The complete structure system is assumed to be com- posed of a group of components that are interconnected.Among them stiffness coupling component is defined as“soft substructure”,the others are divided into many substructure by natural.The vibration mode for each component are determined separately by finite element method under the condition of free-interface.The lower-frequency normal modes are kept and the higher-frequency normal modes are neglected by using a frequency truncation criterion.The “residual flesibility”of these neglected modas is considered.Thus,the assumed modes of the whole structure of Rayleigh-Ritz synthesis are formed.Numerical results show that this method yields better synthesis accuracy,it is effective method for analysis a vibration problem of contains stiffmess coupling substructure system.