In modeling the processes of soil CaCO₃ eluviation and deposition,the Ca²⁺ concentration and the pH value in soil solution must be calculated based on the principles of the soil chemical thermodynamic equilibrium.But it is difficult to calculate them directly,for some parameters(e.g.ionic activity coefficients in soil solutions) are hardly available,which leads to unrealizability of computer simulation of the process.Focused on this problem,this paper presents an algorithm of successive approximation.With known CO₂ partial pressures in soil air and soil temperatures,the algorithm enables computation of Ca²⁺ concentrations and pH values in soil solutions at arid area,which provides a significant supporting technology for computer to simulate the process of soil CaCO₃ eluviation and deposition.This method has been applied and verified in CAEDP model (A model for CaCO₃ eluviation and deposition process in arid area soils).The results show that the method is solidly grounded in theory with results accurate and stable,and process simple and easy to follow.