Density  functional theory (DFT) calculations have been employed to explore the  adsorption properties of pristine and vacancy-defective AlN monolayer  towards SO₂ and NO₂ in terms of their adsorption  energies and electronic structures. The calculations with van der Waals  effect are performed to study the binding mechanism of pristine and  vacancy-defective AlN nanosheet with SO₂ and NO₂.  Three types of vacancy defects are considered including Al-monovacancy,  N-monovacancy and divacancy. The adsorption energies of  vacancy-defective AlN monolayer towards SO₂ and NO₂  are found to be much higher than that of pristine nanosheet. The  equilibrium adsorption sites and configurations of the gas molecules on  AlN nanosheets are determined. The Bader charge transfer analysis  reveals that a considerable amount of charge is transferred from the  vacancy-defective AlN nanosheets to the gases leading to the increase of  adsorption energies. The results of difference charge densities, band  structures, electronic density of states and work function further give  insight into adsorption energies. The dramatical changes of electronic  band structures after gas interaction suggest the excellent adsorption  properties of vacancy-defective AlN monolayer towards SO₂ and NO₂.