It is important to investigate the corrosion resistance and formation mechanism of passive film of Ti-48Al-2Cr-2Nb alloy to improve the resistance to stray corrosion during electrolytic machining. The passive potential of Ti-48Al-2Cr-2Nb alloy was determined by polarization curves, the composition and structure of the passive film were determined by X-ray photoelectron spectroscopy, while the electrochemical properties of the passive film were analyzed using electrochemical impedance spectroscopy and Mott-Schottky theory. The results showed that the passive potential interval of Ti-48Al-2Cr-2Nb alloy in NaNO3 electrolyte was 0.079~1.896 V, and the main components of the passive film were Al2O3 and TiO2 and a small amount of Nb2O5, which was a bilayered porous structure with good corrosion resistance. The passive film had n-type semiconductor properties, and the carrier density decreases with the increase of passive potential. Finally, the schematic diagram of the passive film was constructed. The formation mechanism of passive film of Ti-48Al-2Cr-2Nb alloy was consistent with the theory of phase-forming film, which extended and formed separate phases in a hemispherical model.