The total farmland resource is decreasing at a rather greater speed in recent years in China.The major reasons include:construction land use,ecologic defarming,agricultural struc- ture adjustment,catastrophic failure and so on,which will badly affect the safety of food supplies in our country.To solve the problem of national food security caused by cropland reduction,we advance several dynamic equations on arable land change according to current situation.By the 95% self-sufficiency goal of foodstuff brought forward by the white book Food Issue in China,the insurance quantity of cultivated land should be 1.216 6×10⁸hm2.In our model,we present several recommendations and projects for protecting farmland correspondingly based on numerical simulation,and make a cultivated land output prediction from 2004 to 2010.The result of the study shows:(1) only depending on supplement from reserved cultivated land is not enough to ensure food safety by the yearly decrease rate of farmland at present; and (2) returning arable land to forest and grass plays an important role in the general decrease of cultivated land and their mutual coordination is the key problem to ensure national food security.So we should try our best to reduce farmland occupation for construction purpose,make reasonable ecologic defarming planning,encourage precision farming to increase per unit area productivity and adjust people's dietary structure to reduce meat consumption.