In this study, we have fabricated a high-performance SiO2/β-Ga2O3 (001) MIS capacitor and investigated the effect of low-temperature post-deposition annealing (PDA) on its electrical properties. The SiO2/β-Ga2O3 (001) MIS capacitor exhibits improved gate leakage current and breakdown electrical field after the PDA treatment. The high-frequency capacitance-voltage hysteresis curves reveal that the fixed charge density, as well as the fast and deep near-interfacial trapped charge densities, decreases with the PDA process. We achieved an interfacial trapped charge density as low as 1.6 × 1011 cm−2 eV−1 at an energy of 0.6 eV below the conduction band of β-Ga2O3 (001) through 400 °C annealing.