Abstract: To investigate the repair effectiveness of self-made modified epoxy resin potting adhesives on diagonal cracks, a comparative study was conducted with commercial adhesives. Six specimen beams were subjected to four-point bending tests, analyzing crack propagation, load-bearing capacity, and beam displacement under both monotonic and cyclic loading conditions. The research explored key adhesive properties affecting repair performance from a structural perspective, elucidated the repair mechanism of the modified epoxy resin on inclined cracks, and validated this mechanism through comparative analysis of stirrup strain before and after repair. Results showed that beams repaired with the self-made adhesive had an 8.4 kN increase in initial crack load, a 13.18% reduction in mid-span deflection, and a 70.36% decrease in average strain on both sides of the crack, demonstrating better recovery from fatigue damage. The elongation at the break of the adhesive affects the likelihood of re-cracking, while tensile and bonding strengths are closely related to stiffness recovery and crack expansion. The primary repair mechanism involves forming a bonding layer on the rebar, which, together with uncracked concrete, shares the stirrup’s shear force and enhances structural capacity. A shear capacity formula for repaired beams with web reinforcement was established.