Abstract In this work, the factors affecting the discharge of AZ91 magnesium alloy were investigated during microarc oxidation, such as electrolyte configuration, electrical pulse parameters, and physical properties of the coating in the early discharge. Based on metal and gas electronics, a mechanism of thermal electron emission causing the discharge along the dielectric surface was proposed at the solid-liquid-gas interface. It was in good agreement with the experimental results. Due to the tunnel effect and impurity ionization, the electrons could form an electron current along the surface of the dielectric, i.e. the sediment at the initial electrochemical stage and the metal oxide ceramic coating formed at the post stage of plasma reaction during microarc oxidation. The local high-density filamentary current caused thermal electron emission. Emitted electrons impacted the molecule in gas bubbles which was formed on the surface of the metal oxide coating. Thus the impact ionization happened leading to the formation of plasma. The formation of plasma was influenced by the current density, the defects of local uneven electrical conductivity, the impurity level, the resistance of the coating, and the ionization coefficient of the metal oxide coating.