The frame structure with multi-grid composite walls is a dual seismic structure. The deformation feature of the structure is different from common frame or shear wall, and the natural vibration period of the structure can not be calculated with the method for the frame-shear wall structure. Based on the theory of Timoshenko dual variable beam and collaborative work model, the frequency equation for the structure is derived, and the natural period calculation formula is established combined with the boundary conditions of structure. The approximate calculation method for the basic formula is gived simultaneously. Example analysis shows that: the natural vibration period of the structure can be impacted by the shear stiffness of the lightweight composite wallboard significantly, especially to the high order mode. The influence of the shear deformations of the composite wall to the natural vibration period of the high order mode can not be ignored. The error of approximate calculation method by the basic formula is not large comparing with the analytical solution, and can meet the engineering calculation accuracy requirements.