Abstract: In this paper, the effect of siderite and Na₂CO₃ as additives to pH and Eh of bentonite-groundwater system, critical ionic component in liquid, mineral composition, as well as exchangeable cations was studied by experiment in anaerobic and normal temperature condition.The results showed that adding Na₂CO₃ could not only maintain the system in alkaline range, but also prevent Na-bentonite from transforming into Ca-bentonite.Siderite could lower the Eh effectively, and keep the system in reducing condition and stabilize at -158 mV eventually.The mineral composition of bentonite did not change too much because of adding siderite and Na₂CO₃.The optimal ratio of bentonite, siderite and Na₂CO₃ improving chemical buffer capacity of GMZ Na-bentonite was determined at last.