Abstract: The tests of three specimens of frame joint which are strengthened by enlarging column section and one?comparative specimen under low- cycle reversed load are conducted.It is analysed that the specimens in phenomenon, crack?strength, the load- bearing capacity , stiffness, ductility and so on.The shearing capacity of adding core concrete is?unfavorable when the original column axial pressure is big.The adding core with stirrups has a higher restraint to the original?core than the one without stirrups.The increase in stiffness and energy dissipation is bigger and in ductility is smaller.The?formulas of computing the cracking load of the original core and bearing capacity of the strengthening core are put forward.