The easy-going oxidation of silicon nitride (Si₃N₄) at high temperature greatly hampers its potential applications. Here, we explored the reaction mechanism between β-Si₃N₄ and O₂ via density functional theory (DFT) calculation, which discloses that O atoms are preferentially adsorbed on the top of Si atoms and N₂ starts to be generated as the dominant gas product at 2/3 monolayer (ML) O coverage. The vacancies formed by N₂ removal attract the O adatoms to transfer to the site of the N vacancy, which accelerates the adsorption of O and the formation of Si–O bonds toward the growth of SiO₂ product. The surface oxidation of β-Si₃N₄ (0001) has been clarified by the unambiguous evolution of [SiN_{4 -n}O_n] (n = 0-4) tetrahedrons going through from [SiN₄] tetrahedron to [SiO₄] tetrahedron, providing a deep insight into intrinsic oxidation process of Si₃N₄ ceramic.