The shear strength of the bonding interface between new and old concrete is a critical concern in precast concrete structures and the strengthening of concrete structures. Shear calculation formulas of the bonding interface between new and old concrete given by four codes （GB 50010―2010， ACI 318―19， Eurocode 2， and PCI7th） are compared and analyzed based on the test date of shear tests （containing 439 specimens） of bonding interface between new and old concrete （to be referred to as the interface）. The results show that under the influence of each parameter， the assurance rate of ACI calculation results is the best， and the calculation results of Eurocode 2 fit shear tests best. When the size range of the interface is 300~400 mm， the performance of GB 50010―2010 is best， but both of them are not safe， and the maximum relative error is about 9. According to the parameters analysis results of the database， the shear strength τexp is affected by the size effect and is closely related to the size of the interface along the shear direction. And τexp has a good correlation with the clamping stress ρfy. Based on the analysis results， the shear strength calculation formula is modified according to the superposition calculation theory. The modification includes that the coefficient β is introduced by analyzing the size effect； the friction coefficient μ is modified； the piecewise function of dowel resistance fitted by the database is applied， and its maximum value is 6.6 MPa. It is proved by comparative calculation that the error of the modified formula is the smallest， and the fitting effect is the best when the interface is with or without rebar and has various roughness. Moreover， it can be used as a reference for applications in civil engineering.