The study on tension behavior of casing and dowel joint of square steel tube was carried out by using finite element analysis software ANSYS/LS-DYNA with consideration of geometric nonlinearity, material nonlinearity and contact nonlinearity. On this basis, the effects of inside tube wall thickness, main tube wall thickness, and inserting depth on failure mode, ultimate tensile load and deformation of casing and dowel joint of square steel tube was discussed. The results show that there are three types of failure modes, i.e., bolt failure, inside tube failure and main tube failure, when the joints are subjected to axial tension force. Compare to the joint with the same wall thickness of inside tube and main tube, the reduction of wall thickness of inside tube or main tube will weaken greatly the ultimate tensile load of the joint. The ultimate tensile load of casing and dowel joints is proportional to bolt shear strength, tube wall thickness, inserting depth, and tube edge length. The fruits are useful to the design and application of casing and dowel joints of square steel tube.