The method of molecular dynamics is applied to study the features of the plastic flow of hexagonal magnesium, namely, the evolution of dislocation density of magnesium single crystals of hard and soft orientations during the deformation by compression. The question on the validity of describing the dislocation plasticity at the atomic level by the theoretical dislocation density evolution equations valid at the macroscopic level is considered. The Orowan–Taylor equations, the law of hardening due to interaction of dislocations and their clusters with each other, as well as the differential equation of dislocation dynamics are tested. A relationship between the deformation rate, the defect density, and the Burgers dislocation vectors and active sliding systems is revealed. The simulation results presented in this report are based on the embedded-atom method (EAM) interatomic potential for Mg. Results of calculations of theoretical equations are presented based on the data obtained by molecular dynamics simulation of magnesium deformation by compression.