Abstract: Experiments concerning on mechanical properties of the flat portion, corner portion, virgin steel and full section stub column are performed for the thick-walled cold formed Q345B steel tube with sectional dimensions of 250mm×250mm×9.2mm. Referring to the cold formed technology, the paper analyzes the mechanical characteristics of the flat and corner portion with passes. Strain hardening of thick-wall steel is compared with that in the reported reference. Strength is calculated according to specifications in North American, China and related references for the design of the cold formed steel structural members respectively. Unlike Chinese specification, the foreign specifications tend to be non-conservative and can not be applied directly to the design of thick-walled cold formed steel tube in China. Remarkable differences exist in mechanical properties of flat portion. The increase in full section strength of stub columns is much lower than that of the flat and corner portion.