At present, the main gas-sensing mechanism of oxidized MXene (Ti3C2Tx) is commonly regarded as Schottky barrier modulation, but the influence of surface defects generated by oxidation is ignored and ambiguous. Herein, oxi-dized Ti3C2Tx crumpled spheres (MS) are obtained, accompanying numerous surface defects through thermal oxidation of MS synthesized by ultrasonic spray pyrolysis technology and gas-sensing properties of oxidized MS with Ti3C2Tx/TiO2 crumpled spheres (MT-10-1) without new surface defects are com-pared. It is demonstrated that the significant improvement of the gas-sensing properties of oxidized MS is due to the introduction of Ti atom defects rather than Ti3C2Tx/TiO2 heterojunction in-situ generated by oxidation. First-principles density functional theory calculations show that Ti atom vacancy can greatly improve the adsorption ability of Ti3C2Tx to gases (especially for NO2). Sub-sequently, with the facile oxidability, Ti3C2Tx is utilized as a reductant to assist the reduction of graphene oxide, and Ti3C2Tx/TiO2/rGO crumpled spheres are subtly designed and successfully synthesized for further enhancing the gas-sensing performance. The MG-2-1 sensor achieves a low detection limit of NO2(10 ppb), great NO2 selectivity, and high NO2 response. The clarification of the gas-sensing mechanism of oxidized Ti3C2Tx and the utilization of oxidation of Ti3C2Tx provide a new idea for the application of MXenes.