The dual-phase amorphous/crystalline nanostructured model proves to be an effective method to improve the plasticity of Mg alloys. The purpose of this paper is to explore an approach to improving the ductility and strength of Mg alloys at the same time. Here, the effect of amorphous phase strength, crystalline phase strength, and amorphous boundary (AB) spacing on the mechanical properties of dual-phase Mg alloys (DPMAs) under tensile loading are investigated by the molecular dynamics simulation method. The results confirm that the strength of DPMA can be significantly improved while its excellent plasticity is maintained by adjusting the strength of the amorphous phase or crystalline phase and optimizing the AB spacing. For the DPMA, when the amorphous phase (or crystalline phase) is strengthened to enhance its strength, the AB spacing should be increased (or reduced) to obtain superior plasticity at the same time. The results also indicate that the DPMA containing high strength amorphous phase exhibits three different deformation modes during plastic deformation with the increase of AB spacing. The research results will present a theoretical basis and early guidance for designing and developing the high-performance dual-phase hexagonal close-packed nanostructured metals.