Abstract: The new-to-old concrete interface between the underpinning beam and column is a weak zone of the underpinning joint. Experiments of fourteen specimens with two span ratios and various interface reinforcements were carried out. The results show that the failure mode can be divided into two types:interface failure and shear failure of underpinning beam. The interface failure occurs in the cases of no interface reinforcement or the characteristic value (product of reinforcement yield strength and its area) of the interface reinforcement being less than 2 MPa. When the characteristic value of the interface reinforcement is more than 2 MPa, the interface failure will convert to shear failure of the underpinning beam. Based on the shear-friction model, the calculation formula for calculating the interface bearing capacity was put forward and the involved parameters were determined. In the actual design of the underpinning joints, the amount of the interface reinforcements can be determined according to the bearing capacity of the beam and the interface. The bearing capacity of the underpinning beam is firstly calculated, and then the interface reinforcement is obtain using the interface formula, which ensures the safety of the interface and achieves the optimal design.