Abstract: How the shear force is transmitted between old and new concrete is of common concern. To study the mechanical characteristics of the interface between old and new concrete under pre-stressing forces, the bond behavior of the interface was experimentally investigated through loading tests of composite beams. The considered parameter is the space between the hoop bars. It is observed that, under pre-stressing forces, there is no slip on the interface of the old and new concrete. When the static loads increase to 72% of the ultimate loads, the slip occurs in sheer span region and the local bond failures are observed. A calculated method about stress, strain and slip of the interface is proposed based on the differential equations proposed by Girhammar. The calculated results are in good agreement with the experimental results.