Upon irradiation with mild visible light using a S-1-ethyl-S'-(alpha,alpha'-dimethyl-alpha ''-acetic acid) trithiocarbonate (EDMAT) and a (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TPO) photoinitiator, water-soluble acrylic monomers undergo a rapid and well-controlled RAFT polymerization directly in acidic aqueous solution at 25 degrees C. The effect of visible light on this aqueous RAFT polymerization was investigated by employing a periodic light-on-off process. The results demonstrated that EDMAT was stable in acidic aqueous solution but liable to hydrolysis in alkali solution; its absorption covers a wide visible light wave range of 388-520 nm. This led to a rapid and well-controlled RAFT polymerization of acrylic monomers in acidic aqueous solution at 25 degrees C Simply upon irradiation with mild visible light. The kinetic character of RAFT polymerization at 7 degrees C was quite similar to that at 25 degrees C, which proceeded rapidly and kept living behavior, implying the negligible thermoactivating effect of this aqueous RAFT polymerization. The trithiocarbonate moieties and monomer units did not hydrolyze in such an acidic aqueous solution over the short duration of RAFT polymerization. A periodic light-on-off process leads to a corresponding repeatable periodic polymerization-on-off process. The essentially polymerization-standstill state in the light-off period indicates the negligible concentration of active radicals and the significant slow intermediate fragmentation reaction. Further turning on this visible light leads to another rapid polymerization process with the same kinetic character as that observed in the former light-on process, suggesting that the fragmentation of intermediate radicals in this aqueous RAFT polymerization was significantly activated by this visible light.