Abstract: Biogas slurry, the by-product of biogas engineering comprised of agricultural residue and livestock/poultry waste, is a leading source of agricultural non-point pollution. Biogas slurry is also most difficult to be dealt with in the protection of water environment. To study the safety of the technology of biogas slurry application in paddy fields, 4 treatments of biogas slurry application and two control treatments were designed. The treatments included irrigation 1 000 t.hm^-2 of biogas slurry with zero nitrogen urea application at tillering stage of rice (BS10); replacements of 100%, 200% and 300% of nitrogen urea, respectively, with 211.76 t.hm^-2, 423.53 t.hm^-2 and 635.29 t.hm^-2 of biogas slurry at tillering stage of rice (100%BS, 200%BS and 300%BS). The control treatments were conventional fertilization at 180 kg(N).hm^-2 of nitrogen urea (CF) at tillering stage of rice, and no-fertilization treatment (CK) at tillering stage of rice. Then the changes in total nitrogen, ammonium nitrogen and nitrate nitrogen contents in field surface water were monitored after 1 day, 2 days, 3 days, 5 days and 7 days of biogas slurry application. The same changes were also monitored in percolated soil water at 40 cm and 60 cm depths. The results showed that biogas slurry application at rice tillering stage obviously increased total nitrogen and ammonia nitrogen contents in field surface water. Both total nitrogen and ammonia nitrogen contents increased with increasing amount of biogas slurry application. Ammonia nitrogen was the main form of nitrogen in every field surface water treatment, and the content significantly dropped over time. Compared with the first day after biogas slurry application, total nitrogen decreased by 46.67%73.26% on the third day, where in BS10, 300%BS, 200%BS and 100%BS treatments the reduction in total nitrogen were higher than that in CF treatment by 26.59%, 26.43%, 24.38% and 10.25%, respectively. Also compared with the first day after biogas slurry application, total nitrogen decreased by 69.15%86.43% on the seventh day, where in BS10, 300%BS, 200%BS and 100%BS treatments the reduction in total nitrogen were higher than CF treatment by 13.16%, 12.27%, 11.60% and 5.96%, respectively. Ammonia nitrogen decreased by 47.52%67.60% on the third day, and 75.25%83.73% on the seventh day, respectively, compared with the first day after biogas slurry application. And in BS10, 300%BS, 200%BS and 100%BS treatments, the reduction in ammonia nitrogen were higher than CF treatment by 14.73%, 17.29%, 20.08% and 6.47% on the third day, and by 6.05%, 6.21%, 8.48% and 3.55% on the seventh day, respectively. Thus the first three days after application was the critical period for biogas slurry dissolution in paddy fields. It was also the key period to limit nitrogen loss through surface drainage which in turn reduced agricultural non-point source pollution. Compared with conventional fertilization treatment, there was no obvious effect of BS10, 300%BS, 200%BS and 100%BS treatments on total nitrogen and ammonia nitrogen contents of percolated soil water at 40 cm soil depth, however, total nitrogen and ammonia nitrogen contents obviously increased at 60 cm soil depth. Total nitrogen under BS10, 300%BS, 200%BS and 100%BS treatments was higher than that under CF treatment by 0.37 mg?L1, 0.67 mg?L1, 0.13 mg.L^-1 and 0.23 mg?L1 at 60 cm soil depth, respectively, on the seventh day after treatment. Ammonia nitrogen under BS10 and 200%BS treatments was higher than that under CF treatment by 0.02 mg.L^-1 and 0.36 mg.L^-1, respectively. The impact of treatment 100%BS on total nitrogen and ammonia nitrogen contents was weakest both in field surface water and in percolated soil water at three days after biogas slurry application. Duplicate values of total nitrogen concentration of percolated soil water at 40 cm and 60 cm soil depths under each treatment had considerably large range of variations. Analysis of variance of total nitrogen concentration in percolated soil water showed no significant difference at 0.05 level between 40 cm and 60 cm soil depths. Combined with rice production safety, it was recommended to limit biogas slurry application to less than 211.76 t.hm^-2 during tillering stage.