A composite connection joint for section steel plug-in welding of concrete prefabricated piles was designed to improve the connection performance of concrete prefabricated piles. The shear and flexural experiments of thirteen composite joints and pile bodies of composite reinforcement concrete prefabricated square piles with section side lengths of 300mm, 450mm and 600mm were conducted to compare the shear and flexural bearing capacities, deformation capacity and failure characteristics of the section steel plug-in welding composite joints and pile bodies. The results show that the shear failure mode for the pile bodies is the oblique section shear failure in the flexural shear section, and for the joints is the oblique section shear failure in the flexural shear section and the steel bar pier heads being pulling off at the joint. The main flexural failure mode for the pile bodies is the tensile fracture of the steel bars in the tensile zone of the section, and for the joints is the steel bar pier heads being pulled off at the joint. For the shear specimens, many vertical cracks are observed in the pure flexural section, along with an obvious oblique abdominal shear crack in the flexural shear section. While for the flexural specimens, the vertical cracks in the pile bodies are numerous and evenly distributed, with bifurcation phenomena. The shear and flexural bearing capacities, and deformation capacity of 300mm and 450mm square pile joints are close to those of the pile bodies. For the 600 mm square pile joint, adding anchoring rebars to the end plate at the joint can effectively enhance the flexural bearing capacity and deformation capacity of the joint.