SnO2 quantum dots (QDs) were successfully synthesized via a microwave-assisted reaction of a SnO2 precursor in an aqueous solution using a microwave system. The morphology, structure and photocatalytic performance in the degradation of methylene blue (MB) were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction spectroscopy, UV-vis absorption/reflectance spectroscopy and electrochemical impedance spectroscopy, respectively. The results show that the SnO2 QDs synthesized at a pH value of 5 exhibit an optimal photocatalytic performance with a MB degradation rate of 90% at 240 min under visible light irradiation due to their easier adsorption of pollutants, higher visible light absorption and lower electron–hole pair recombination.