Abstract: Silver nanoparticles (AgNPs) could enhance the photocatalytic reaction significantly owing to its surface plasmon resonance (SPR) effect. However, AgNPs are usually unstable without the ligands and their preparation process always requires organic chemicals as the reducing agents. In this work, humic acids (HA), widely existent in environmental water samples, were used as both reductant and ligand for in situ synthesizing and stabilizing AgNPs under simulated sunlight. The results of photocatalytic experiments indicate that the as-prepared AgNPs show excellent activity for the degradation of tetrabromobisphenol A (TBBPA), a typical organic halogenated pollutant in water. With AgNPs generated in the mixture of 1 mg/L HA and 2 mmol/L Ag+, 74.9% TBBPA was degraded in 1 h. This degradation efficiency is much better than that of commercial AgNPs. Through controlling the reaction conditions, neutral pH was found to be beneficial for the degradation of TBBPA, which would lower the requirements of equipments and reaction conditions in practical applications. By the inhabitation test of active species, singlet oxygen (O2), hydroxyl radical (·OH) and superoxide anion (O2·-) generated via the SPR effect of the in situ formed AgNPs under illumination, were simultaneously identified to be the active species in the degradation of TBBPA. Therefore, the AgNPs with high photocatalytic activity are in situ synthesized by using cheap HA under mild conditions, exhibiting potential for applications in photocatalytic degradation of organic pollutants in wastewater.