Effects of organic acids on the activity of aluminum, calcium and magnesium in acid soils were investigated under pH buffer solution by adding exogenous low-molecular-weight organic acids into acid soils pretreated by CaCO3. The results showed that in the solution of pH 4.5, the soluble Al and the exchangeable Ca was increased significantly while the exchangeable Al was decreased significantly after the addition of oxalate, citrate and malate whether CaCO3 was amended or not; When CaCO3 was added, the exchangeable Mg was all increased under oxalate, citrate and malate treatment. The ability of organic acids to promote the aluminum dissolution by complexation follows the order as citrate > oxalate > malate, which is consistent with their stability constants of aluminum - organic complexes. On the other hand, the pretreatment of CaCO3 caused the augment of soluble Al and the descent of exchangeable Al. The results of two-factor analysis of variance indicated that the effects of organic acids were overwhelming predominance on the soluble and exchangeable Al, Ca and Mg by complexing or precipitating, while CaCO3 only had significant effects on soluble Al, exchangeable Al and Ca dominantly by competing and exchanging between Al and Ca, Mg because of the control of pH buffer solution. On the whole, the addition of low-molecular-weight organic acids made the significant increase of active Al and the slightly higher of active Ca and Mg. The roles of organic acids in the actual acid soils need further evaluation from the effects of dissolution of cations.