<p>Excessive ammonia nitrogen has been demonstrated to cause a serious hazard to water environments. Bacteria performing simultaneous nitrification and denitrification (SND) can be effective biological instruments to remove ammonia nitrogen completely from effluents. For the first time, <i>Pseudomonas oleovorans</i> QS-7 with SND function, isolated from the biogas treatment system of a pig farm, was found to efficiently remove ammonia nitrogen. Through the determination of key enzymes and functional genes related to the nitrogen metabolism of strain QS-7, combined with nitrogen balance measurements of the nitrogen metabolic process, it was speculated that the SND pathway of the novel strain is NH<Stack> ₄ ⁺ </Stack>→NH₂OH→NO<Stack> ₂ ⁻ </Stack>→NO<Stack> ₃ ⁻ </Stack>→NO<Stack> ₂ ⁻ </Stack>→NO→N₂O→N₂. QS-7 exhibited 98.6% ammonia nitrogen removal and a maximum ammonia degradation rate of 9.2 mg/(L·h) at 18 h in 100 mg/L ammonia nitrogen solution. This strain also has a certain capacity to remove nitrate and nitrite nitrogen; the maximum removal efficiencies were 54.22% and 73.93%, respectively, in systems with 100 mg/L of nitrate or nitrite nitrogen as the sole nitrogen source. Nitrogen metabolic balance analysis for QS-7, using ammonia (100 mg/L) as the sole nitrogen source, demonstrated that assimilation (56.1%) is the main mode of nitrogen removal, followed by conversion to N₂ (43.6%). Meanwhile, NO<Stack> ₂ ⁻ </Stack> was not detected, and almost no NO_<i>x</i> was produced, which indicates that the nitrogen removal process of QS-7 is environmentally friendly. The optimal environmental conditions for QS-7 were found to be sodium citrate as the carbon source, C/N=10, pH=7.0, 150 r/min, and 30 °C. The above results indicate that QS-7 may provide a material and conceptual basis for the advancement of SND technology.</p>