<p>Micro-arc oxidation (MAO) technology is widely used to enhance the surface properties of metals. However, the formation process of the MAO layer is still unclear, and the film-forming mechanism still needs to be further investigated. In this paper, a mechanistic study of the MAO process was carried out, and a simulation model of the temperature field coupled with the thermal stress field and the electrochemical field was established to analyze the effects of temperature and thermal stress on the morphology of the film layer as well as the distribution law of the film layer growth. Simulation and experimental results show that the cooling effect during MAO film formation is mainly concentrated in the 100 µs after the formation of the discharge channel, the maximum temperature gradient is located on both sides of the discharge channel, and the unbalanced rate of temperature decrease in each part of the channel is the main reason for the ‘crater-like’ holes on the surface of the film layer. The thermal stresses are mainly concentrated in the discharge channel region, providing the electron avalanche’s driving force. The growth rate of the film thickness is relatively stable, with an average thickening rate of 2.54361 µm/min, and the film thickness on the surface of the workpiece shows the distribution characteristics of thick around and thin in the middle.</p>