Abstract: This study investigates the buckling instability of ultra-high strength steel thin-walled columns using axial compression tests on C-shaped open section specimens with varying stiffening configurations. The research identifies that: the specimen without web stiffeners primarily exhibit local-distortional buckling interaction, whereas those with V-shaped web stiffeners show either distortional buckling or distortional and global buckling interactions. Finite element modeling was employed to validate experimental results and to examine the impact of stiffening forms on the buckling modes and load capacity of thin-walled C-section columns. The study evaluates the effectiveness of the direct strength method in current design standards, integrating both experimental and computational data. The "Technical Standard for Cold-formed Steel Structures" (draft for approval) is generally applicable for columns with distortional-global buckling interaction but tends towards slightly unsafe predictions. The American standard, focusing on local-global interaction, demonstrates greater variability. The revised calculation method separately accounts for local-distortional and distortional-global buckling interactions, offering increased reliability and less variability, thus making it more suited for ultra-high strength steel thin-walled open section axial compression columns.