Abstract: The grain microstructure and deformation mechanism of nano polycrystalline Cu-Ni alloy were studied by molecular dynamics simulation to explore the influence of average grain size on the mechanical properties of nano polycrystalline Cu-Ni alloy. The results show that with the gradual increase of strain, the stress presents a cliff like decline and then presents a serrated stable fluctuation, which is due to the release of stress concentration. The grain size at the critical point of average flow stress change is 5.86 nm, when the grain size is greater than 5.86 nm, the deformation mechanism shows Hall-Petch effect; when the grain size is less than 5.86 nm, the deformation mechanism shows the inverse Hall-Petch effect.When the grain size is 4.65 nm, the yield strength and young’s modulus of the material reach the peak value of 17.2 GPa and 60.0 GPa respectively, and the mechanical properties reach the best.