A lab-scale two-stage UASB-A/O combined process was used to investigate the effect of free ammonia（FA）and free nitrite acid（FNA）on the short-cut nitrification of municipal landfill leachate rich in ammonia nitrogen（NH4+-N）on the premise that the COD and nitrogen were removed simultaneously.The denitrification and methanogenesis were conducted in UASB1 simultaneously for the partial removal of TN and COD.The effluent COD of UASB1 was further removed in the UASB2 by methanogenesis.Subsequently，the denitrification of NOx--N in the returned sludge using the residual COD and the short-cut nitrification was achieved in the A/O reactor.The experiment experienced three stages in 104 d：the stable short-cut nitrification，the damaged short-cut nitrification，and the recovered short-cut nitrification.The results showed that when the lowest FA was above 3.1 mg·L-1 the stable short-cut nitrification could be achieved and NH4+-N removal efficiency，NO2--N accumulation ratio and TN removal efficiency were 99%，95% and 86%，respectively.When the FA was lower than 0.6 mg·L-1 with sufficient alkalinity and extended aeration，the short-cut nitration was damaged rapidly and NO2--N accumulation ratio decreased to 29% due to the weak inhibition of low FA on NOB.FNA was lower than 0.011 mg·L-1 at previous two stages，which led to the failure of inhibition on NOB.While FNA was increased considerably in the 3rd stage（the highest FNA was 0.414 mg·L-1）by decreasing pH，the short-cut nitrification was recovered soon and maintained and NO2--N accumulation ratio increased to 92% owing to the cooperative effect of FA and FNA.In brief，FA and FNA were important factors to achieve and maintain the short-cut nitrification of the municipal landfill leachate.