Abstract: In order to investigate issues of leaching malachite with ammoniacal solution, thermodynamic equilibriums of CuO-CO₂-NH₃-H₂O system is studied, obtaining a series of equilibrium diagrams. The results show that OH^- replaces gradually to form precipitate when pH is increased. The order of stable solids from low to high pH value is as follows: azurite, malachite and copper hydroxide. PH plays a decisive role in copper leaching. At low pH, H⁺ competes with Cu²⁺ to combine with NH₃, which leads to a low concentration of copper-ammonia complex in solution; at high pH, OH^- competes with NH₃ to combine with Cu²⁺, which makes copper in solution convert to copper hydroxide precipitate. When the pH value is between 8 and 11, the competitive effects of H⁺ and OH^- are relatively weak. Most ammonia form complex with copper, and is the predominant species. Besides, the calculated theoretical optimal pH for leaching malachite is between 9.3 and 9.4. There is a roughly linear relationship between the total concentration of copper and nitrogen under this condition.