Magnesium (Mg), a lightweight metal material, is constrained in its applications due to poor plasticity and low strength at high temperatures. Graphene (Gr) possesses a large specific surface area and high strength, making it an ideal reinforcement for improving the mechanical properties of materials. A molecular dynamics (MD) simulation was employed to investigate the mechanical behaviors of single-crystal Mg and Gr/Mg composites under compressive loading. Through the analysis of stress-strain curves, atomic structure diagrams, and dislocation distributions, the microscopic deformation mechanisms of single-crystal Mg and Gr/Mg composites under compressive loading were explored. Additionally, the influence of factors such as the number of Gr layers, loading strain rate, and temperature on the mechanical properties of materials was studied. Results reveal that single-crystal Mg exhibits anisotropic characteristics under compressive loading. Addition of Gr enables the activation of difficult-to-initiate slip systems in the Mg matrix due to grain refinement. This leads to stress release and difficulty in initiating twinning deformation. Near the Gr interface, defects such as dislocations and twins nucleate and proliferate, effectively transferring the load to Gr, thereby elevating the average flow stress during the plastic deformation stage of the composites. Furthermore, the Mg matrix restricts the folding and bending of Gr, leading to an enhancement in material toughness. As a result, when the Gr/Mg composite is compressed along the [0 0 0 1] crystal direction to a strain of 0.35, the Gr remains intact without fracture. Dislocations in Gr/Mg composite materials cannot penetrate the Gr layer, thus suppressing Mg matrix damage. Increased dislocation lines can resist compressive plastic deformation. In composites featuring multiple layers of Gr, the yield stress, yield strain, and average flow stress during the plastic deformation stage increase with the number of Gr layers. Additionally, the yield strain is higher when Gr layers are separated compared to being stacked. Within the temperature range of 10 K-600 K, the elastic modulus and yield stress of Gr/Mg composites decrease with increasing temperature. However, the strain rate has a minor effect on the elastic modulus and average flow stress during the plastic deformation of Gr/Mg composites. Nonetheless, increasing the strain rate can enhance the yield stress and yield strain of the composites.