The plasma electrolytic oxidation (PEO) is an innovative method to endow surface multifunctionalities on Al, Mg, Ti, Zr, Nb, Hf, Ta, and their alloys. While its quality is determined by many factors such as power supply regime, electrical parameters, electrolyte and substrate materials, etc. Herein, from the perspective of PEO process parameters and coating structure optimization, the growth rule and optimization strategies of the PEO coating are revealed by changing the power supply regime, electrical parameters, electrolyte composition and substrate materials. We will first introduce the influence of the power supply output regime on the microstructure of coating, including output mode, electrical parameter and “soft” spark discharge. Then, we highlight the effect of various electrolyte compositions (such as special ions, particles, dopants, etc.) on coating microstructure. Finally, the effect of substrate composition including reaction thermodynamics, alloying element, and metallurgical state on coating microstructure is reviewed, as well as the challenges to improve the coating quality/versatility and future development prospects are summarized.