Random defects on concrete surfaces are a key factor affecting the bonding property of concrete–epoxy mortar interfaces. In this study, an epoxy–acetone solution was employed to pretreat such defects on concrete surfaces, and resin concentrations of 20, 40, 60, and 80 wt% diluted by acetone were used. The fracture mechanical properties and propagations of the fracture process zone (FPZ) of 14 concrete–epoxy mortar composite specimens were analysed using digital image correlation technology and the four-point bending test. The results revealed that there was a great influence of resin concentration on the fracture properties of the concrete-epoxy mortar interface. Precoating solution with an appropriate resin concentration (20 ∼ 40 wt%) could effectively delay crack formation at the interface and markedly improve the bearing capacity, stiffness, ductility, FPZ length, and energy release rate of the specimens. The precoating solution with a resin concentration of 40 wt% exhibited the optimal repair effect on the concrete surfaces. However, the improvement from the pretreatment with high resin concentrations (60 ∼ 80 wt%) on the fracture properties was less than satisfactory. The toughening mechanism of this precoating solution was analysed through scanning electron microscopy, which revealed that the concrete surface could be effectively repaired and improved by the precoating treatment with an appropriate resin concentration, which in turn enhanced the interfacial connection performance between epoxy mortar and concrete.