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SnO2‐x nanocrystals were prepared using a simple hydrothermal method by controlling the proportions of Sn4+/Sn2+ precursors. Due to the doping of Sn2+, the oxygen vacancies could be introduced into SnO2 nanocrystals, and their concentration can be controlled by the amount of doping Sn2+. The structural characteristics of SnO2‐x nanocrystals was investigated by X‐ray diffraction, Fourier transform infrared spectra, Raman spectra, UV‐vis diffuse reflectance, photoluminescence spectra and X‐ray photoelectron spectroscopy. The results reveal that the bridging O‐vacancies are dominant in the pure SnO2 while the in‐plane O‐vacancies play a key role in the SnO2‐x. Meanwhile, the higher Sn2+ concentration, the more structural disorder was produced. According to XPS analysis, the presence of oxygen defects will change the electronic structure of SnO2‐x nanocrystals, which will lead to the valence band moving up and the smaller band gap. The rich oxygen vacancies, the structural disorder and the narrow band gap should greatly improve the visible light harvesting, and result in the excellent photocatalytic activity. [ABSTRACT FROM AUTHOR]