Abstract: Gradient-grain structure materials exhibit many excellent mechanical properties, such as high strength, high toughness and fatigue resistance, by constructing microstructures from nanocrystals and fine crystals to coarse crystals in the interior of the materials. Engineering materials will inevitably suffer fatigue and fracture during long-term service, which seriously threatens the service safety and service life of materials. It has been reported that the nanocrystalline materials have the characteristics of intergranular fracture, and the fracture resistance is related to the grain size, but the gradient-grain structure materials have a complex grain size distribution, and the fracture mechanism needs to be further revealed. Therefore, based on the crystal plastic finite element method, a cohesive element is implanted on the polycrystalline grain boundary of the finite element model. The single tensile properties of homogeneous-grain structure copper and gradient-grain structure copper are simulated respectively, and the effect of precast crack on crack propagation of gradient-grain structure materials is studied. The results show that the proposed crystal plastic constitutive model combined with the grain boundary damage mechanism can effectively simulate the plastic deformation and crack propagation of gradient-grain structure materials. The gradient-grain structure materials exhibit the gradient distribution of stress and plastic strain under uniaxial tensile deformation. On the one hand, due to the effect of grain size, the surface nanocrystalline strength is high although the flow stress in the fine-grained region of the matrix is relatively low. In addition, due to the difference in strain hardening capacity, although the nanocrystalline region shows a lower plastic strain, the fine-grained region shows a higher plastic deformation capacity. Therefore, the gradient-grain structure effectively optimizes the synergistic effect of strength and toughness through the difference of strength and strain hardening ability, thus enhancing the ability to resist crack propagation. The precast crack in the nanocrystalline region has a great influence on the strength of gradient-grain structure materials, so restraining the crack initiation in the nanocrystalline region is conducive to the safe service of grain structure materials.