Abstract: Microstructure evolution in the crack-tip region under cyclic loading is a critical factor influencing fatigue damage behavior. However, effective simulation approaches to address how crack-tip stress concentration and non-uniform deformation drive grain boundary migration and grain size evolution remain lacking. In this paper, a dislocation-density-based crystal plasticity model and a phase-field model describing grain-scale microstructure evolution are integrated to develop a crystal-plasticity phase-field model for simulating crack-tip microstructure evolution under cyclic loading, revealing the grain structure evolution driven by stress concentration and its regulatory mechanism on the crack-tip stress field. Methodologically, a dislocation-density-based crystal plasticity constitutive model is employed to describe anisotropic plastic deformation, while the phase-field model uses multiple order parameters to represent the polycrystalline structure. The elastic strain energy and the stored energy arising from dislocation accumulation serve as the driving forces for microstructure evolution, enabling the simulation of strain-energy-driven grain boundary migration and grain growth. Through comparative simulations of specimens without initial crack, with single initial crack and with symmetrical initial cracks, the influence of stress concentration on microstructure evolution is systematically analyzed. The results show that the high stress concentration at the crack tip induces directional grain boundary migration. Under the influence of single initial crack, a grain morphology extending towards the crack tip is formed. Under the influence of symmetrical initial cracks, a large area of grain refinement zone is formed. In the specimen without an initial crack, the grain morphology remains relatively uniform. A grain refinement zone appears near the crack tip in the specimen with initial crack, resulting in a microstructure morphology featuring coexisting fine and coarse grains, and a phenomenon of abnormal grain growth occurs. As the microstructure evolves continuously, grain boundary migration significantly relaxes the peak stress at the crack tip by altering the local stress field. The proposed crystal-plasticity phase-field model in this paper provides an effective simulation tool for characterizing the interaction between the crack-tip stress field and microstructure evolution.