The two-phase copolymerization of acrylamide (AM) and anionic comonomer, sodium 2-(acrylamido)-2-methylpropanesulfonate (NaAMPS), in a poly(ethylene glycol) (PEG) aqueous solution was investigated. An unusual soluble-insoluble soluble phase transition was found. The effects of the monomer ratio, PEG concentration, initiator concentration, salt concentration, and pH value of the reaction media on the phase transition behavior were studied. As the NaAMPS molar fraction increased or the PEG concentration decreased, the insoluble soluble phase transition first appeared and then disappeared. However, all of the aqueous two-phase polymerization systems underwent a soluble insoluble soluble phase transition with increasing initiator concentration. The results of critical conversion and copolymer composition indicated that there were two competitive effects governing the phase separation, that is, the solubility enhancement of a polyelectrolyte and Debye-Huckel screening effect of an anionic monomer. The insoluble soluble phase transition disappeared at higher salt concentration. A mechanism for the unique soluble insoluble soluble phase transition was proposed, which was ascribed to the synergistic effects of the polymer concentration, solubility enhancement of a polyelectrolyte, and screening effect of an anionic comonomer. On the basis of this mechanism, the phase transition was successfully tuned by varying the pH value of reaction media.