Interface properties and bias temperature instability (BTI) are the two critical issues that severely restricted the performance and reliability of SiC metal-oxide-semiconductor (MOS) devices. In this work, we simultaneously improved interface properties and BTI in 4H-SiC MOS capacitors by modifying SiC/SiO2 interface via ternary H-Cl-N mixed plasma post-oxidation annealing (POA). Results showed that H-Cl-N mixed plasma POA improved the oxide insulating properties, reduced density of interface traps, and improved the BTI both at low temperature and high temperature. The modification of interface was achieved by enriching H, Cl, and N elements at and near the interface, which not only led to a smoother interface, but also could bond with interfacial traps. The synergistic effects of H, Cl, and N in reducing traps at and near the interface were due to the conversion of the trap levels to or outside the lower half of the SiC bandgap, thereby suppressing electron trapping. Density Functional Theory calculations further indicated that the Cl and N could passivate mobile ions and Si-Si bonds. N passivation and combined N and H passivation were stable and effective in passivating Si-Si bonds and reducing oxygen vacancies.