According to the principle of thermodynamics, thermodynamic phase diagrams are calculated and drawn, the possibility, direction and physicochemical conditions of the two chemical reaction processes of TSR reaction and dissolution of carbonate rocks by H₂S are determined, and the direct reduction of CaSO₄ (or SO₄²⁻) to H₂S at different temperatures is obtained. It shows that when CaCO₃ is at the boundary between precipitation and dissolution in geological system, a small amount of acidic fluid will make the precipitated CaCO₃ dissolved, and when the concentration of Ca²⁺ and CO₃²⁻ increases, a new equilibrium of precipitation and dissolution will be achieved. When the depth of dissolution of CaCO₃ is about 1000 m, H₂S achieves the best effect. Only long-term and repeated TSR reactions can produce sufficient acidic fluid (H₂S), which is the necessary condition for dissolution modification of carbonate reservoirs to achieve obvious results.