The numerical simulation studies on flexural behavior of casing joint of square steel tube were 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 tube wall thickness, tube edge length, and inserting depth on failure mode, ultimate flexural capacity and deformation of casing joint of square steel tube was discussed. The results show that there are two types of failure modes, i.e., inside tube yield failure and outside tube shear failure, when the joints are subjected to lateral concentrated load. Ultimate flexural capacity and rigidity of casing joint of square steel tube increased with the inserting depth increasing. The ultimate flexural capacity of the joint is proportional to tube shear strength, tube wall thickness, inserting depth, and tube edge length. The fruits are useful to the design and application of casing joint of square steel tube.