Abstract: During the process of ultra-precision cutting of polycrystalline material workpieces, grain size significantly affects dislocation generation, propagation, and interaction, which affects the mechanical properties of the material. To study the influence of grain size on cutting performance, the Voronoi and the molecular dynamics methods were combined. Using the open-source software LAMMPS and Atomsk, the cutting process of polycrystalline copper with varying grain sizes was simulated. Variations in cutting force, microscopic surface unevenness, and internal plastic deformation of the workpiece were studied through dislocation analysis. The study showed that the average value of cutting force gradually increases with the decrease of grain size, the average temperature and the microscopic surface unevenness also tend to gradually increase during the cutting process. At the same time, plastic deformation inside the workpiece triggers dislocation reactions, and nanoscale grains can effectively inhibit the dislocation activity.