The vibro-acoustic characteristics are very most important in evaluating the living comfort and the service life of build? ings. Therefore， it is of great significance to optimize the vibro-acoustic characteristics of buildings. This research has considered the building system as a coupled fluid-structural system： the structural part is a rectangular structure made of concrete material， and the fluid part is the cavity inside the rectangular structure filled with compressible air. In this work， a hybrid mode synthesis strate? gy has been developed to analyze the vibro-acoustic characteristics of buildings： on the one hand， the branch mode is introduced to develop the hybrid mode synthesis theory， and the corresponding reduced model has been built based on the fixed interface mode， branch mode and free interface mode， which greatly improves the analysis efficiency of the acoustic-structural system. On the other hand， based on the reduced model， an optimization strategy by minimizing the Sound Pressure Level （SPL） in the cavity is devel? oped， and typical focus include the quantitative relations between the SPL and the thickness of the wall along with floor-wall con? nection stiffness. This work aims at providing an efficient method for optimizing vibro-acoustic characteristics of buildings.