Abstract-In order to provide a new approximated method for the preliminary design, tub-in-tube structure is simplified as a double cantilever beam system with compatible horizontal displacement at each floor along one of orthogonal direction of structure. According to the linear elastic assumption, the fluctuation equations of tubes, the equilibrium of floor and displacement compatibility conditions, the spectrum of dynamic response of structure such as displacement, rotation angle, bending moment and shear force of two tubes are expressed by a transfer vector with 6 dimensionless elements. The equations to solve seismic response, natural frequency and mode shape of structure are finally derived by taking into account the boundary condition of structure. This method has 8 parameters in each storey and is suitable for structure with different storey parameters. With the aid of Fast Fourier Transform, it can provide time history solution for displacement, rotation angle, bending moment and shear force of every tube at the top and bottom of each storey. During the preliminary design, the designer can use this method to realize a satisfied design efficiently.