Referring to the SMART model, the organic layer model was developed to describe acidification of forest organic layer. The organic layer model and the modified SMART model were combined to evaluate forest soil acidification more properly. This new model is called two layers model. Considering ecosystem processes concerned to acidification or neutralization, ex. nitrogen mineralization, nitrification, nutrient uptake and creation of organic acid, forest soil acidification was predicted for 100years using the two layers model. Model prediction revealed that nitrogen mineralization rates and nitrification rates had great effects on forest soil acidification and that organic layer neutralized acid deposition.