In order to improve the bond quality of new to old concrete, a new construction process of scattering dry gravels onto the surface of self compacting concrete was put forward to replace the process of roughening the surface of old concrete. The dry gravels were considered as reinforcing particle, and rock failure process analysis (RFPA) code was used to study the failure process of slant shear specimens with different adhesion strength and reinforcing particles under static loading. The results of slant shear test of scattering dry gravels onto the surface were compared with that of the numerical simulation. The numerical simulation analyzed the failure characteristics of the specimens with different bond strength. When the adhesion strength of bonding surface is less than 30MPa, the strength of slant shear specimens of concrete with different strength grades is close, and is depended on the adhesion strength. Otherwise the strength of slant shear specimens increases with the concrete strength grades, and keeps stable around the overall axial compressive strength of the same concrete specimen at last. The size of single reinforcing particle almost has no influence on the elastic modulus and strength of slant shear specimens. The larger size of multiple reinforcing particles is better to improve the strength of slant shear specimens. The bonding surface is no longer the weakest position, and compares well with the actual experimental results. Compared with the process of roughening the surface of old concrete, the process of scattering gravels with the diameters of 520mm onto old concrete surface can improve the slant shear strength by 107%.