This paper explored the densification effect on the plasma electrolytic oxidation (PEO) film coated on Al substrate under the transient self-feedback control mode. Film compactness and microstructure uniformity were carried out by different surface analysis and testing instruments. The results showed that the high-frequency carrier waveform kept a reasonable control on plasma discharge intensity during different stages, inhibited concentrated plasma discharge, prevented the mullite formation, promoted gamma ->alpha Al2O3 phase transition sufficiently, and refined the alpha-Al2O3 grain distribution. These advantages basically stop the crack and defect generation and improve obviously the PEO film compactness and microstructure uniformity.