The phase separationphenomena or cloud points (CP) behaviorof nonionic octylphenol polyoxyethylene ether with oxyethylene glycol ether unit of 15 (OP-15) were investigated in aqueous micellar solutions containing high concentrations of inorganic salts including NaCl, KCl, MgCl2, CaCl2, BaCl2, AlCl3, FeCl3, Na2SO4and Na2SiO3. The change in CP induced by salts and its mechanism are discussed theoretically. In aqueous solutions containing high concentration of salt, for the first time the linear relationship between CP and salt concentrationiswellestablishedtocharacterizethedepressionofCP.Basedonpseudophaseseparatio model, thermodynamic parameters are estimated and adopted to further describe the phase separation process and the interactions between OP-15 molecules or the OP-15/water interactions. Thermodynamic parameters including standard Gibbs energy change (ΔG0 CP), enthalpy change (ΔH0 CP) and entropy change (ΔS0 CP) indicate that the additions of different salts result in the change in entropy-driven nonspontaneous process of phase separation. These data, which are from the compensation temperatures and the change of molar heat capacity, also prove that different ions induce the change in the interactions between surfactant molecules or the surfactant.water interaction and thus affect the depression of CP. These findings help with understanding the effect of inorganic salts, especially the high concentration of salt, on CP of nonionic surfactant and then broadening the application of nonionic surfactant based on CP such as the extraction of biomaterials in biological samples derived from animals or plants, etc.