Abstract: 15N-labeled technique was used to study the fixation, transformation, fate and utilized efficiency of N in the farming -stockbreeding-biogas ecosystem of rice areas. The results showed that the crude protein digestibility of the ensilaged milk vetch by pig was 53.76%, the recovery rates of ensilaged milk vetch N by the pig feces and urine were 39.36% and 24.71%, respectively. The recovery rate of N of pig feces and urine was 97.9% for biogas fermentation. The content of hydrolyzable N in biogas fertilizer was 2.6 times higher than before the fermentation. The combined application of milk vetch, biogas fertilizer and chemical fertilizer can promote N translocation to rice grain to increase the yield. The residual N of fertilizer in soil and gaseous loss N in combined application of milk vetch, biogas fertilizer and chemical fertilizer was 1.0～1.5 times higher and 1.6～7.2 lower than that in the treatment of chemical fertilizer alone, respectively. The comprehensive economic benefit in the farming-stockbreeding-biogas ecosystem increased by 300%～400%, compared to that of milk vetch-early rice-late rice system, and ecological and social benefit were also significant.