Abstract: Tests on the bentonite-Beishan underground water system with additives of Na₂CO₃ and siderite were carried out under high temperature and low oxygen condition. Variations of pH and Eh of system, zeta potential and viscosity of colloid, exchangeable cations and mineral composition of bentonite, as well as concentration of key cations in liquid along with the additive dose were investigated. The results show that adding Na₂CO₃ could not only maintain pH value within the range between 7.84 and 8.99, but also prevent Na-bentonite from transforming into Ca-bentonite on account of improving components of exchangeable Na⁺ to 60 mmol/100 g. At meantime, the stability of colloids is strengthened. Siderite could reduce the Eh value of system to -124 mV effectively and keep the system in reduction state eventually. The mineral composition of bentonite does not change too much because of adding siderite and Na₂CO₃. Under the conditions of high temperature and low oxygen, the optimum mass ratio of bentonite, Na₂CO₃ and siderite to enhance the chemical buffer performance of Na-bentonite is 400∶12∶160.