Abstract: The pollution of groundwater by NO₃^- in the decommissioned mining area in an in-situ leach uranium mine is being paid more and more attention. The denitrifying bacteria by domesticating the sludge taken from the decommissioned mining area in an in-situ leach uranium mine in North West of China were obtained. An up-flow fixed-bed denitrifying bacteria bioreactor was designed. The effects of pH, the ratio of carbon to nitrogen and HRT on the removal of NO₃^--N from the polluted groundwater in the decommissioned mining area in the in-situ leach uranium mine by the denitrifying bacteria bioreactor were investigated. The results show that when pH, the concentration of NO₃^--N and HRT are set to 6.50, 1 000 mg/L and 2.3 h, respectively, the removal of the NO₃^--N amounts to 97%, the NO₃^--N degradation rate is 388 mg/(h•L) and the capacity of the bioreactor is 0.35 m³/(h•m³). When pH, the concentration of NO₃^--N and HRT are set to 6.50, 550 mg/L and 1.4 h, respectively, the removal of the NO₃^--N amounts to 96% and the capacity of the bioreactor is 0.62m³/(h•m³). The bioreactor should work on the operation conditions that pH is 5.00-8.00 and the ratio of carbon to nitrogen is 0.6-0.8.