Based on statistical data (1990–2008, and 2011), the effect of crop-livestock structure on environmental capacity of soil nitrogen was studied at a farmland system in Xiangtan county of Hunan province, China. The soil-surface balance method was used to calculate environmental capacity of soil nitrogen. The results showed that the arable area of farmland system in Xiangtan county increased by 7 180 hm2 from 1990 to 2008, the crop area decreased by 26 530 hm2, and the multiple cropping index declined 58.06%. The crop yields increased by11.57%, while the breeding stock of pigs climbed to 111.94%. The nitrogen supply capacity was 540.94 kg/hm2, the assimilative capacity was 236.40 kg/hm2, the nitrogen carrying capacity of soil was 304.54 kg/hm2 at farmland system. It indicated that the nitrogen was surplus. The study demonstrated that the large amounts of nitrogen of farmland system were provided by fertilizers.