Abstract: A heterotrophic nitrification and aerobic denitrification bacterium strain named XK51 was isolated and purified from nitrogen-contaminated shallow aquifer porous media. The strain was identified as Pseudomonas Putida by morphological observation, physiological and biochemical characteristics and 16S rDNA gene sequence analysis. The results showed that strain XK51 was a facultative denitrifying bacteria, which could achieve high denitrification efficiencies under aerobic or anaerobic conditions. The maximum and average denitrification rate were 27.3 mg/(L·h) and 4.4 mg/(L·h), the nitrate removal efficiency was 95.3%. XK51 also had high heterotrophic nitrification capacity, and its maximum and average nitrification rate were 4.2 mg/(L·h) and 1.4 mg/(L·h), then the ammonia removal efficiency was 98.5%. The optimum carbon source was trisodium citrate, and the optimum growth temperature ranged from 28 to 35 ℃, with a preferred value of 30 ℃. The optimum cultivating pH value ranged from 6.5 to 8.0, with a preferred value of 7.0. Strain XK51 could simultaneously carry out heterotrophic nitrification and simultaneous nitrification-denitrification with no obvious nitrite and nitrate accumulation during cultivation. It has potential engineering application value in nitrogen-containing wastewater treatment and nitrogen pollution remediation of groundwater.