The influence of Si content on the formation process and properties of PEO ceramic coating on the surface of Al Si alloy was studied. SEM, XRD and EDS were used to analyze the microstructure and phase of the ceramic layer, and the heat insulation performance and the hardness of the were tested by using the thermal insulation temperature measuring device and the 500MVDTM digital display. The results showed that in the early plasma electrolytic oxidation, silicon atom and a second phase Al3CuNi will hinder micro-arc discharge, inhibiting the formation of an aluminum oxide film, reducing the density of the film; with the matrix increased silicon content, primary silicon and eutectic silicon content increases in Al-xSi alloy, the phenomenon that silicon particles stacked appears, the more difficult plasma discharge, the growth rate of the coating is reduced, the content of silicon dioxide and silicon impurities in the ceramic layer will be increased, heat temperature also will increase, hardness decrease .