A one-dimensional unsteady ignition and combustion model is established for the pulverized magnesium particles in a spherical cloud. The behavior of ignition and combustion of magnesium particle cloud is numerically simulated. The result shows that the ignition of particle cloud occurs at the boundary of particle cloud first, then the initial of which bifurcates into two flames, one of which propagates into the particle cloud, and the other moves away from it. Finally, the inner flame disappears because of O2 depletion, and only the outer flame, which maintains and controls the combustion of magnesium particle cloud, exists at the outside of it. The flame propagation velocity accelerates, while the flame temperature decreases during the process of the inner flame going into the magnesium particle cloud. The effects of the interior and the environmental parameters on the ignition and combustion of the magnesium particle cloud were analyzed. With the increase in the particle concentration, the ignition delay time increases slightly, but the propagation velocity of the inner flame becomes faster, and the steady particle cloud flame sphere is enlarging. With increasing initial temperature of the particle cloud, the ignition delay time canbe reduced significantly, the propagation of inner flame speeds up, but the size of steady particle cloud flame sphere keeps almost constant. The effect of ambient temperature on ignition and combustion of particle cloud is complicated. The higher the ambient temperature, the shorter the ignition delay time, however, the propagation velocity of inner flame becomes slower, and the size of the steady particle cloud flame sphere changes very insignificantly. Both the size of particle and the temperature of radiation source have great influences on the ignition and combustion of particle cloud. The smaller the particle size or the higher the temperature of radiation source, the shorter the ignition delay time of particle cloud, the faster the propagation velocity of inner flame, and the bigger the size of steady flame sphere. The results of numerical simulation are in good agreement with experimental data published in the literature.