Abstract: An amphiphilic block copolymer（mPEG-S-S-PS） with a hydrophilic poly(ethylene glycol) methyl ether (mPEG) block, a hydrophobic polystyrene (PS) block and a disulfide linker was synthesized via atom transfer radical polymerization (ATRP). The structure and composition of mPEG-S-S-PS were characterized by nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Through solvent dispersion and microscopic observation, phase inversion of emulsions stabilized by mPEG-S-S-PS was studied. Thanks to the amphiphilic property of mPEG-S-S-PS and the insolubility of PS block in cyclohexane, the diblock copolymer self-assembled to form micelles composing of PS cores and mPEG coronas in water and to form inverse micelles composing of mPEG cores and PS coronas in cyclohexane. Therefore, for cyclohexane-water biphasic system with equal volumes, W/O type emulsions were preferred for copolymer initially dissolved in cyclohexane, while for copolymer initially dissolved in water, O/W type emulsions were obtained. However, for toluene-water biphasic systems, W/O type emulsions were always obtained no matter where the copolymers were dissolved because toluene was a non-selective and good solvent for both PS and mPEG blocks. Based on redox responsiveness of mPEG-S-S-PS copolymer, the addition of reducing agent DTT or GSH could induce the phase inversion of toluene-water emulsion from W/O type to O/W type. The catastrophic phase inversion was obtained in this system by adjusting the oil-water volume ratio. When the fraction of water volume (φ_w) was less than 0.7, the emulsion was W/O type. However, when φ_w≥0.7, a catastrophic phase inversion occurred where the system shifted in the opposite direction and O/W type emulsion was obtained. These results will have potential applications in the field of controlling drug release.