Soil surface acidity, as one of the most important characteristics of soil solids, is very Closely related with adsorption, floculation and other interface phenomena, and acid-base titration curve technique is widely used to describe the acidity. A model to describe the acidbase titration curves of variable charge soils was established on the assumption that all the H⁺ or OH^- added into soils were buffered by soil surface groups M-OH₂^+1/2, M-OH^-1/2, and M-o^-3/2 and equations concerning the relationship between equalibrium suspension pH and amount of acid or base added were derived according to the principle of mass balance and charge balance. The acid and base rirrarion curves and buffering curves were modelled using the model on computer and the results showed that Ka₁ has great effect on both acid and base titration curves, but Ka₂ inflenced the base migration curves only wuh no effect on acid titration curves. The model could well describe the acid titration curves of 3 variable charge soils. The model could provide soil acid intensity (Ka₁ and Ka₂), soil acid capacity (St), buffering capacity pH of soil acid and base at different pH, and greatest buffring capacity pH of soil. The acid capacity (St) was mainly determined by the content of soil clays and soil special surface, the acid intensity was mainly determined by the content of iron oxide. Buffering capacity of soils to acid and base was correspondance with K₁ and K₂. The experimental results of acid titration of 3 variable charge soils, red soil, lateritic red soil and latosol, could fitted well with the model. The model will be helpful in understanding the acidification mechanism, adsorption process and other surface processes.