The tests on thirteen specimens of casing joints of square steel tube were conducted to investigate the flexural behavior of the joints. And numerical simulation studies on that were carried out by ANSYS/LS-DYNA. On this basis, effects of tube wall thickness, tube edge length, and inserting depth on failure mode, ultimate flexural capacity and deformation of the joints were discussed. The results show that there are two types of failure modes, i.e., inside tube yield failure and outside tube shear failure. Ultimate flexural capacity and rigidity of the joints increased with the inserting depth increasing. The ultimate flexural capacity is proportional to tube shear strength, tube wall thickness, inserting depth, and tube edge length.