The work aims to study the mechanism of the overall plasma oxidation on the surface of dissimilar metals connected part based on dielectric breakdown theory. Based on the existing experiments, a layer of overall plasma oxidation film was prepared on aluminum alloy and dissimilar aluminum alloy welded part. The timing and selectivity mechanism of overall plasma oxidation on the surface of dissimilar metals were verified by dielectric breakdown experiment. Based on dielectric breakdown theory, the mechanism of solid state dielectric breakdown on different material surfaces was deeply analyzed to study the growth mechanism of overall ceramic coating on dissimilar metal surfaces. The physical/chemical activity of different materials affected the plasma oxidation process. Metals with high chemical activity preferentially underwent ceramization reaction, thus causing non-balanced growth of ceramic membrane. There are timing and selectivity in the overall plasma oxidation process of dissimilar metals, which are related to the chemical activity of materials.