The interface will be the weakest part after the rehabilitation of old concrete substrate; however, the durability of the interface has been unclear. To fill this gap, this study designed three typical interfaces to simulate those in field projects, i.e., the concrete-repair concrete interface, the concrete-repair cementitious grout interface, and the concrete-repair cementitious grout interface under flexural loading. An improved salt ponding test and a self-designed loading device for simulating the chloride contamination environment coupled with the loading were proposed. Results showed that as the water to cement ratio (w/c) of concrete decreased, the difference in chloride profile was also reduced in each part of the repair system, which presented no obvious difference when the w/c was 0.4. The chloride ion concentration was distributed in an asymmetric inverted V shape in all cases along the perpendicular direction of the interface. Besides, the influenced area of crack at the interface in the prefabricated concrete substrate was 10–20 mm, while that in the cast-in-place concrete substrate was 5–10 mm, along the direction perpendicular to the interface. The crack widths between 0.08 and 0.11 mm posed no great threat to the chloride diffusion at the interface under flexural loads. The present study suggests that the interface is the weakest part, for both mechanical properties and durability of a repaired concrete system. A novel model was proposed to describe the in-depth mechanisms based on complicated microstructure characteristics and particle–particle interactions at the interface.