Adsorption and diffusion of Mg atom on the monolayer black phosphorus (P) and its structural stability with the increasing Mg concentrations were investigated using density functional theory. The adsorption energy was -1.09 eV for the Mg adsorbed on the monolayer black P. The Mg ions showed an anisotropic diffusion behavior on the monolayer black P with diffusion barriers of 0.08 and 0.57 eV along the zigzag and armchair directions, respectively. The monolayer of black P can keep the lattice structure stable forming as the Mg0.5P. These results proved that the monolayer black P can be used as a potential anode for Mg-ion batteries.