Abstract: A 3D microscopic unit-cell model of sand/resin was built based on the failure of sand/resin matrix fracture and sand/resin interface debonding. This model was employed to study the microscopic stress characteristics in resin sands, the damage of resin-bonded bridge, and the effect of microstructure (resin content, sand, sand particle size distribution, and the effective cross-sectional area ratio bonding bridge) on the tensile strength of sand/resin. A fracture mode based on energy mechanism i.e., cohesive behavior method was used to describe the debonding of sand/resin interface, and the extended finite element method (XFEM) was used to capture the matrix damage and crack propagation. The numerical results show that the proposed model can explicitly depict the microscopic fracture behavior of sand/resin and explain their fracture mechanisms. The valuable information involving the influence of the resin content, sand size, sand size grading, and the effective cross-sectional area ratio on the tensile strength (S_t) under the tensile loading is provided. This work can provide theoretical guidance for the resin sand optimization design.