Abstract: Livestock waste poses a great pollution to the cropland and even results in eutrophication of environment if agricultural farming-animal husbandry ecosystem is not soundly planned. In this study, a mathematic model to estimate livestock supporting capacity of cropland was built based on nitrogen cycling in form of agricultural farming-animal husbandry ecosystem (N-LSCM). The parameters necessary to this model are pig feed protein equivalent, nitrogen excretion equivalent and nitrogen consumption per unit yield of crops. In order to test this model, the actual data of Sichuan province of southwest China were used as a representative situation, and the calculation results showed that in 2006, the agricultural scale of livestock and poultry was reasonable, it did not pose a threat to the environment, and there was still an increase potential in livestock of 22?419?574 pigs (Duroc×Landrace×Yorkshire, slaughtering weight 100 kg). The model can also be used for risk assessment of livestock-environment.