The separation and categorization of the basic “global (G), distortional (D), and local (L)” mode classes are necessary for systematical analysis and design of thin-walled members. This paper proposes a new set of basic mode definitions based totally on the force characteristics, which are opposite to the conventional deformation shape-based definitions. In contrast to the current general beam theory (GBT) and constrained finite strip method (cFSM), the proposed G, D, and L classes span the entire deformation space of the thin-walled member, and are strictly orthogonal to each other with respect to the stiffness of the member. Buckling mode decomposition and identification according to the proposed definitions are realized based on finite strip models of thin-wall members. Numerical examples show consistent applicability of the proposed method to open/closed polygonal/curved cross-sections. Further, effects from shear and transverse extension deformations can be reasonably accommodated in the GDL classes, thus no further separation is needed.