Nitrogen and oxygen doping modification has a significant effect on improving the ammonia nitrogen adsorption and recovery performance of biochar. However, the enhanced adsorption behavior and mechanism of ammonia nitrogen at nitrogen-oxygen doped sites remain to be further clarified. Starting from the molecular atomic scale and relying on the density functional theory (DFT) method, the adsorption behavior and mechanism of ammonia nitrogen in different forms of nitrogen and oxygen were studied in depth by constructing biochar ammonia nitrogen adsorption systems with different nitrogen and oxygen doping structures. The calculation results showed that the adsorption energy between the undoped carbon skeleton structure and NH+4 was 4.65kJ/mol. After nitrogen and oxygen doping, the adsorption effect was increased by 2.59 to 14.81 times and the adsorption effect was significantly improved. At the same time, judging from the influence of doping position, there was little difference in the adsorption of the same group at different doping positions. The adsorption energy was changed between 1.09kJ/mol and 8.49kJ/mol. Therefore, the NH+4 adsorption mechanism of nitrogen oxygen groups in different forms was further analyzed and discovered. In nitrogen-oxygen single doping, the adsorption effect of nitrogen oxide and carbonyl groups was the strongest, which was the result of the combined effect of hydrogen bonding and van der Waals attraction. The adsorption effect of the co-doping of nitrogen and oxygen groups was increased by 4.7~9.8 times, and the adsorption capacity was between nitrogen and oxygen single doping. However, the co-doping of carbonyl and nitrogen oxide groups caused competitive adsorption, which weakened the adsorption effect of the co-doped structure on NH+4. Finally, the ammonia nitrogen adsorption efficiency experiment of different nitrogen and oxygen modified biochars effectively, which verified the rationality of the above theoretical calculation and analysis results. The research results can provide an important theoretical basis for the directional construction of nitrogen and oxygen heteroatom structures on the surface of biochar and its application in ammonia nitrogen adsorption and recovery.