Abstract: Plasma electrochemical oxidation (PEO) is a surface modification technology to form ceramic coatings on magnesium alloys. However, its application is limited due to its defects. This work reports a novel preparation of in-situ sealing of PEO coatings by four-layer voltage and sol addition. The morphology and structure were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffractometer (XRD). Image-Pro Plus 6.0 was used to determine the porosity of the coating, which was decreased from 8.53% to 0.51%. Simultaneously, the coating thickness was increased by a factor of four. The anti-corrosion performance of each sample was evaluated using electrochemical tests, and the findings revealed that the corrosion current density of coatings (i_corr) of the samples were lowered from 9.152 × 10^–2 to 6.152 × 10^–4 mA·cm⁻², and the total resistance (R_T) of the samples were enhanced from 2.19 × 10⁴ to 2.33 × 10⁵ Ω·cm². The salt spray test used to simulate the actual environment showed that corrosion points appeared on the surface of the coating only at the 336 h. In addition, the mechanism of PEO self-sealing behavior was described in this article.