Abstract: Due to the large difference in mechanical properties between the faceplates and core, the interlaminar shear deformation of the stiffened composite sandwich structure is obvious, and the interlaminar shear stress has a significant impact on the buckling characteristics of the structure. In addition, the transverse shear deformation between stiffeners and plates will also significantly affect the buckling characteristics of the stiffened sandwich structure. Therefore, it is necessary to develop a mechanical model that can accurately calculate the transverse shear stresses between the faceplates and core and between stiffeners and faceplates to study the buckling characteristics of composite stiffened sandwich structures. Thus the sine type global-local higher-order shear deformation theory is derived. This theory meets the conditions of in-plane displacement, transverse shear stress continuity and free surface conditions, and the number of unknowns is independent of the number of layers of stiffened sandwich plates. Based on the theory, the discrete Kirchhoff triangular element (DKT element) is used to construct the sinusoidal global-local triangular plate element (SGLT), and the accuracy of the model is verified by two numerical examples. Then the buckling characteristics of metal stiffened sandwich plates and composite grid stiffened plates under various geometric, material parameters and boundary conditions are evaluated. The numerical examples show that the established finite element model can accurately predict the buckling behaviors of stiffened sandwich structures. And compared to the three-dimensional (3D) finite element model, the established model has high computational efficiency.