Eight sets of 50 new-old concrete interface specimens were subjected to double-sided direct shear tests using the interface processing method, lateral constraint form, concrete strength, and interface size as variables. The shear failure mode and shear strength slip curve of the interface were obtained, and the size effect law of the interface shear strength was analyzed. The conversion coefficient of the ultimate stress size effect of the new-old concrete double-sided direct shear specimens was fitted, and an expression for calculating the shear bearing capacity was established. The results show that in the direction of shear force, there is a significant size effect phenomenon in which the shear ultimate stress at the interface between new and old concrete decreases with the increase of interface height. The size effect of cracking stress, crack through stresses, and slip is not significant. Applying lateral constraints can increase the interface crack through stress and ultimate stress. Applying preloading stress can signifi-cantly increase the crack-through stress of the specimen and effectively limit the ultimate slip of the specimen. Under the condition of no lateral constraint, the chiseling form has a significant impact on the ultimate stress. Compared with the whole poured concrete specimen, the mechanical chiseling of new-old concrete with the same strength reduces the double-sided direct shear ultimate stress to below 0.22 of the whole poured specimens, while the reduction coefficient of the manual chiseling specimen is between 0.34 and 0.37. Under lateral constraint conditions, compared to the rough interface with chiseling, the regular tooth groove specimen has prominent advantages in cracking stress, crack through stress, and slip performance. The strength reduction coefficient under constraint can be increased to more than 2~2.5 times that of the unconstrained specimens under the same conditions, and the maximum strength can reach 0.97 times that of the cast-in-place specimens. The increase in the strength of new-old concrete can increase the crack through stress and ultimate stress of specimens, but the extent of improvement is limited.