To design and develop magnesium-based nanostructured alloys with excellent mechanical properties, a model of nanocrystalline magnesium with random grain orientation was constructed by the Voronoi geometry method. The compression simulation of nanocrystalline magnesium under different conditions was realized by molecular dynamics software. The simulation results were analysed by visualization software. The results show that with the increase of temperature, the grain size changes from refinement to fusion growth; the results show that the compression speed affects the time of grain refinement. With the increase of compression speed, the atoms in the grain still keep the original structure, only the atoms at the edge of the grain move, the grain refinement occurs later, the yield strength increases, the ultimate strain decreases, and the elastic modulus increases; the shift of atomic position in nano polycrystalline magnesium makes it easier to form FCC structure, resulting in Shockley incomplete dislocation, which is in positive proportion to the growth rule of FCC structure.