Abstract: There are two types of calculation methods for the shear strength of the existing new-to-old concrete interface without anchor bars: considering normal stress and without considering normal stress. These methods lack theoretical support and need to improve the calculation accuracy. A unified calculation formula that takes into account both normal stress and zero normal stress has not been reported. According to the shear test failure phenomenon and the new-to-old concrete interface mechanism, the failure mechanism of a new-to-old concrete interface without anchor bars is analyzed. Based on the three-dimensional energy yield criterion, on virtual work principle and, on plastic limit analysis theory, the theoretical calculation formula of shear strength of new-to-old concrete interface without anchor bars is derived. The comprehensive coefficient considering the influence of normal stress and of roughness is introduced. The theoretical formula is modified using three-dimensional curve fitting based on the existing shear test data of new-to-old concrete interface without anchor bars. A unified calculation formula of shear strength of new-to-old concrete interface without anchor bars is obtained. Based on 27 groups of compressive (tensile) shear test specimens and on 30 groups of pure shear test specimens of the new-to-old concrete interface without anchor bars, different methods are used to calculate the shear strength, and a comparative analysis is conducted. The results show that: for the compressive (tensile) and shear test specimens, the deviation between the calculated values and experimental values of the method is generally less than 15%, and the calculation accuracy of the unified formula is higher than that of the existing calculation methods considering normal stress. For pure shear test specimens, the deviation rates of the calculated values of the method proposed are mostly within 20%. Except for the large deviation rate of low strength concrete specimens, the calculation accuracy of the unified formula is much higher than that of the existing calculation formula without considering the normal stress. The unified calculation method has a sufficient theoretical basis and is simple in computation. It can be applied to compressive (tensile) shear composite members and pure shear members. Meanwhile, the calculation accuracy is better than that of the existing calculation methods.