Aqueous two-phase systems (ATPSs) consist of two immiscible aqueous phases where the mole fraction of water in both upper and bottom phases is higher than 0.8. ATPS has been widely used in many different fields since its discovery in 1896. However, easy control of phase transition is highly desired in many cases, which is still a challenge. Herein, we report the first work for the reversible switching of transition between two-phase ATPSs and single phase solution by light irradiation. It was found that azobenzene-based ionic liquids and inorganic salts in water systems could be reversibly switched between two-phase ATPS and homogeneous solution at ambient conditions by alternatively UV and visible light irradiation. Mechanism studies reveal that cationic isomerization of the azobenzene-based ionic liquids and size change of their micelles by UV and visible light irradiation is the main driving force for such reversible phase transition of ATPSs. This unique phase behavior has bridged the gap between ATPSs and chemical reactions, which is useful to create new chemistry in ATPSs. As an example of applications, highly efficient and selective aza-Michael reactions were performed in this novel kind of switchable ATPSs, which was well integrated with heterogeneous product separation, and the phase components could be easily reused. This work opens a new way to conduct reactions efficiently with easy separation process.