Abstract: According to the principles of simultaneous equilibria and mass equilibria, CuInSe₂ (CIS) were prepared by total aqueous process at normal pressure and room temperature. A series of thermodynamic equilibrium equations were deduced for the complex system of Cu⁺-In³⁺-Se^2--NH₃-L^2--Cl^--H₂O (T=298 K), and the equilibrium curves of lgc-pH were drawn. It was found from these equilibrium diagrams that[NH₃]_T primarily affected the[Cu⁺]_T concentration,[L^2-]_T influenced the[In³⁺]_T concentration, and[Cl^-]_T had impact on both. The highest precipitation coefficient was obtained at pH 7.0-8.5. As a result, based on the principles of simultaneous equilibria and mass equilibria of the Cu⁺-In³⁺-Se^2--NH₃-L^2--Cl^--H₂O system (T=298 K) and used total aqueous process at normal pressure and room temperature, CIS nanoparticles can be synthesized by adjusting the pH value to 7.0-8.5, and their size is about 20 to 50 nm.