Ag quantum dots (Ag QDs) modified BiOBr ultrathin nanosheets have been prepared via the reactable ionic liquid assisted solvothermal method. The Ag QDs was evenly dispersed on the surface of BiOBr nanosheets with the diameter less than 5 nm. The obtained Ag QDs/BiOBr materials displayed greatly enhanced photocatalytic activity for the degradation of antibiotic agent ciprofloxacin (CIP) and tetracycline hydrochloride (TC) under visible light irradiation. Through the various characterizations, the relationship between structure and activity was studied in details and some new insights of the key role of Ag QDs for the enhanced photocatalytic activity was presented. Different from the roles previous reported such as electron injection, plasmon resonance energy transfer and Schottky junction, the modified Ag QDs could active the molecular oxygen via the hot electron reduction under the visible light irradiation in this system. And the Ag QDs worked as adsorption centers, charge separation centers and photocatalytic reaction centers, contributed to the increased photocatalytic activity. This study provided an approach to improve the molecular oxygen activation for pollutant removal, selective organic synthesis or donating abundant hot electrons for CO2 photoredution. (C) 2016 Elsevier B.V. All rights reserved.