Abstract: Research on the fracture behavior and coalesence of defective rocks is of great theoretical and practical significance for rock engineering in mines, tunnels and underground chambers. Here, the notched semi-circular bending specimens containing holes of different sizes were filled and reinforced, respectively, and the effect of filling size on the fracture behavior of sandstone under three-point bending loading was investigated. The results show that the proportion of grout significantly affects the mechanical properties and damage process of sandstone specimens. Compared with the intact specimens, the fracture toughness and fracture energy of the specimens decrease by 82.18% and 84.30%, respectively, when the filling diameter reaches 16 mm. During the loading process, internal cracks first appear at the upper end of the straight-cut groove, but with the increase of the filling diameter, the cracks gradually turn to expand along the interface perpendicular to the loading end, and finally overshot failure occurs. In addition, the horizontal displacement of the upper end of the straight-cut groove generally shows mutation behavior, and the mutation area is significantly enlarged with the increase of filling diameter, accompanied by more active acoustic emission behavior. Finally, the effect of water-cement ratio on the mechanical behavior of the filling-reinforced specimens is briefly discussed. With the increase of water-cement ratio, the failure angle of the upper end of the notch increases significantly, the crack tends to develop along the interface, and the overall mechanical properties of the specimens are further weakened.