In this study, the microspheres assembled from SnO2 nanocrystals with abundant oxygen vacancies were synthesized by hydrothermal method using citric acid as a chelating agent and controlling the molar ratio of Sn4+/Sn2+. The SnO2 nano-microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse reflectance spectroscopy. The results show that under the acidic hydrothermal conditions and the chelating effect of citric acid, the tin dioxide nanocrystals aggregate to form microspheres; at a Sn4+/Sn2+ molar ratio of 3∶7, the size of the microspheres is the smallest and the overall dispersion is good. The presence of the oxygen vacancies will extend the light absorption range to visible light; thus, the sample shows a strong visible light catalytic efficiency and complete degradation of methyl orange within 8 min.