Metal-free atom transfer radical polymerization (ATRP) was successfully achieved in aqueous media for the first time. Polymerization of poly(ethylene oxide) methyl ether acrylate (PEGA(480)) was well controlled ((sic) < 1.40) under visible light irradiation using tetrabromofluorescein (Eosin Y) as catalyst and pentamethyldiethylenetriamine (PMDETA) as electron donor. A validated kinetic model was developed to investigate the process of photoredox catalytic cycle via reductive quenching pathway. Experimental and simulation results showed that electron donor not only had an important influence on the ATRP activation, but also participated in the ATRP deactivation. Furthermore, the effects of water content, catalyst concentration, and degree of polymerization on the polymerization were studied thoroughly by a series of experiments. Good controllability of the polymerization regulated by light on and off confirmed the high degree of temporal control. The livingness of the chains was proved by a successful chain extension experiment. Both experimental and simulation techniques were used to study aqueous metal-free ATRP, which provided a promising method to synthesize polymers in the absence of metal and organic solvent.