A one-dimensional unsteady magnesium particle cloud ignition model with finite influencing sphere is established. The behavior of ignition of magnesium particle cloud is numerically simulated. The result shows that when the reaction is speeded up on the surface of magnesium particle, the temperature of the particle phase rises rapidly up to ignition temperature, while the surrounding air is much slower in temperature rising than particles; the gas temperature rising is unconspicuous in the whole sphere in the ignition process, albeit it is significant near the particle surface. The effects of the interior parameters and the environmental parameters on the ignition of the magnesium particle cloud are analyzed. With the increase of particle concentration, the particle cloud becomes easier to be ignited, and reduction in its ignition time delay can be seen. However, when the particle concentration has increased to some specific extent and its further increase will be adverse to the ignition of the particle cloud. The influence of the environmental pressure on the ignition of particle cloud is insignificant, and the ignition performance of the particle cloud almost keeps constant in a range of 1-5 atm. The oxygen concentration in the gas phase also has a weak effect on the ignition performance of particle cloud, but when the oxygen concentration is very low, the effect will significantly increase. The particle size, the initial temperature of the gas/particle and the radiant source have all great influences on the ignition performance of the particle cloud. Small particle and high temperature are helpful for speeding up the ignition process. The tendency obtained by numerical simulation coincides well with that of the experimental results from the literature.