To improve the mechanical properties of Mg alloys and broaden their application fields, high performance Mg matrix nanocomposites have received more and more attention nowadays. Therefore, the research on the basic mechanical properties and strengthening mechanism of new Mg matrix composites at nanoscale has important theoretical and practical significance. The mechanical properties of pristine single-layer graphene nanoplatelets (GNPs) and single-side and double-side nickel-coated GNP (Ni-GNP, Ni-GNP-Ni) reinforced Mg composites (GNP/Mg, Ni-GNP/Mg, Ni-GNP-Ni/Mg) are studied under uniaxial tension by molecular dynamics (MD) simulations. Meanwhile, their tensile properties are also compared with those of double-side nickel-coated GNP with vacancy defects (Ni-defected GNP-Ni) and double-side nickel-coated multilayer GNPs (Ni-nGNPs-Ni) reinforced Mg-based composites. The simulated results show that the mechanical properties of Mg matrix composites are improved significantly by the addition of GNPs. Compared with single crystal Mg, the tensile strength and elastic modulus of GNP/Mg nanocomposites at 300 K and 1×10⁹ s^-1 are increased by 32.60% and 37.91%, respectively; while the tensile strength and elastic modulus of Ni-GNP-Ni/Mg composites are increased by 46.79% and 54.53%, separately. In addition, there is a larger increase in the elastic modulus and tensile strength but a smaller increase in the fracture strain for Ni-defected GNP-Ni/Mg composites, while there is a larger increase in the tensile strength and fracture strain but a smaller increase in the elastic modulus for Ni-GNP/Mg composites as compared with those of GNP/Mg composites. The elastic modulus, tensile strength and fracture strain of Ni-GNP-Ni/Mg composites decreases with increase in temperature, showing a temperature softening effect, but the variation in the elastic modulus of the composites is insensitive to temperature. With increasing of the layers or volume fractions of GNPs in Ni-nGNPs-Ni, the elastic modulus, tensile strength and fracture strain of the composites are all increased significantly, and the composites show excellent comprehensive mechanical properties. It is concluded that the main strengthening mechanisms for Ni-GNP-Ni/Mg nanocomposites are strong interface bonding, effective load transfer from the Mg matrix to the Ni-GNP-Ni and dislocation strengthening by analysis of the evolution of atomic structure.