Abstract: The stability of the dual system with different heights has been studied in this paper. First, the dual system in which the flexural substructure is higher and the shear one is lower is analyzed, while the buckling equation of this dual system is derived, and the analytical solution for the critical load is proposed. According to the interaction between the upper and lower parts of the dual system, the approximate formula for the critical load has been given. Compared with the analytical solution, the approximate one not only has good accuracy but also is much easier to be applied. Secondly, the dual system in which the flexural substructure is lower and the shear one is higher is studied. According to the research, we may find that there are two different buckling modes due to different vertical loads, one is the upper part of the structure buckling first, the other is lower part buckling first. The rule to determine which part of the structure will buckle first under certain kind of vertical load is given, meanwhile the formula for the critical load is proposed.