In this study, hydrogen peroxide (H2O2) cosolvent, which was dissolved into supercritical-phase carbon dioxide fluid (SCCO2), is employed to passivate excessive oxygen vacancies of the high-mobility tungsten-doped indium oxide without any essential thermal process. With the detailed material analysis, the internal physical mechanism of the cosolvent effect or the interaction between the cosolvent solution and supercritical-phase fluid is well discussed. In addition, the optimized result has been applied for the thin film transistor device fabrication. As a result, the device with SCCO2 + H2O2 treatment exhibits the lowest subthreshold swing of 82 mV/dec, the lowest interface trap density of 8.76 × 1011 eV-1 cm-2, the lowest hysteresis of 47 mV, and an excellent reliability and uniformity characteristic compared with any other control groups. Besides, an extremely high field-effect mobility of 98.91 cm2/V s can also be observed, while there is even a desirable positive shift for the threshold voltage. Notably, compared with the untreated sample, the highest on/off current ratio of 5.11 × 107 can be achieved with at least four orders of magnitude enhancement by this unique treatment.