Formation mechanism of the micro-arc oxidation spark discharge is always a research problem; so far there isnt a theoretical model for analyzing the spark discharge formation which is widely recognized. Based on analyzing the reaction phenomena and the coating feature in the different periods of the red spark discharge, the formation model of the red spark discharge was established. The spark discharge formation mechanism is shown that it causes the spark discharge formation that the electron bombards the gas layer wrapping the coating in the infancy of red spark discharge stage; at the later period the area of strong electric field generates the spark discharge where the coating of excessive polarization turns out easily.