A modified tin dioxide/aluminum oxide was prepared by co-precipitation, and its photocatalytic activity was measured. Tin dioxide modified by adding aluminum oxide had a higher photocatalytic activity than other photocatalysts in this study, especially at pH 7. A mass balance for carbon with UV-254 nm/SnO2/Al2O3 system at pH 7 indicated that the amount of the intermediate increased with reaction time, but decreased after 120 min of photodegradation. The results reveal that the photocatalytic carrier, Al2O3, not only increased the amount of 4-nitrophenol removed but also promoted mineralization and destroyed 4-nitrophenol, which was degraded to other simple organic substances or inorganic substances.