Nitrogen oxides (NO_x), as one of the main atmospheric pollutants, seriously affect environmental safety and human health. Selective catalytic reduction (NH_3-SCR) is the most effective treatment method for NO_x removal at present, especially low-temperature SCR denitrification technology, which has very important application prospects and research significance. The research showed that VPO/TiO_2 catalyst had excellent low-temperature SCR denitrification activity. The adsorption of NH_3 and NO on V_2O_5 and (VO)_2P_2O_7 surface was simulated by DFT, and the reaction mechanism of VPO catalyst was studied at the micro level. The results showed that NH_3 exhibited stable chemisorption of Brønsted acid and Lewis acid on the surface of V_4P_4O_22H_14 cluster, and the adsorption effect on Brønsted acid sites was more stable. NH_3 adsorption effect on the surface of catalyst was excellent because of the hybridization of 2p orbital of N, 2p of O and 3d electron orbital of V. NO also showed chemisorption on the surface of V_4P_4O_22H_14 cluster. Due to the small amount of electron transfer, which was significantly lower than the adsorption energy of NH_3, NO could not strongly compete with NH_3 molecule. In addition, NO showed stable chemisorption on Brønsted acid and Lewis acid on the surface of V_6O_20H_10 cluster. The adsorption on Brønsted acid sites was more stable. The E-R mechanism and L-H mechanism of VPO/TiO_2 catalyst reaction mechanism were explained.