This paper systematically reviews the researches on the heat effect of a single steady-state microdischarge during plasma electrolytic oxidation (PEO) and explaines the causes for the appareut changes of microdischarge population which may be related with the evolution of the type and quantity of the disintegrated gas bubbles formed in the interface between the electrolyte and the substrate. The temperature field inside a microdischarge was estimated by virtue of a cylindrical channel model of microdischarge, and the temperature effect in the coatings adjacent to the microdischarge channel was calculated through the heat transfer equation of point heat source during the decay process of microdischarge, which provides a preliminary theoretical analysis for the types and crystalline states of different phase compositions (MgAl2O4,Mg2SiO4, etc.) contained in the coating formed on the magnesium alloy.