Abstract: A system consisted of two-stage UASB-Anoxic/Oxic reactor(A/O) was used to study the short-cut nitrification of the mature municipal landfill leachate with low COD and high ammonia concentration.The COD concentration of influent leachate was 4.3 g/L, and the ammonia concentration of influent leachate was 2.8 g/L, so the ratio of C/N was only 1.5.The uasbl methane production and the denitrification happened at the same time.The concentration of uasbl effluent COD was removed further in the uasb 2.Then nitrification took place in the A/O reactor.The results of the experiment showed as follows:First of all, when we did not add up carbon source to the system, the COD was used as carbon source of denitrification by the first UASB.It was short of carbon source of denitrification in the A/O reactor.Then, when we added up carbon source to the first shelve of A/O system to advance the denitrification, because the denitrification can produce alkalinity, which can complement alkalinity by nitrification consumed, and maintained pH at a suitable scope.By this way, we successfully achieved an stable short-cut nitrification.Short-cut nitrification with 87%of nitrite accumulation efficiency took place.But when we did not add up carbon source to the system, the nitrite accumulation of rate was only 20%.The ammonia concentration of effluent leachate was 10 mg/L.The ammonia removal efficience was 99.6%.But when we did not add up carbon source to the system, the ammonia removal efficience was 92%.