Abstract: Based on V₂O₅-WO₃/TiO₂ catalyst, the size distribution, morphology, element compositions and chemical compositions of fine particles at the output of SCR-DeNO_x system were analyzed, and the formation mechanism of fine particles were discussed based on DRIFTS systematically. The results show that great quantities of particles can be formed during the process of SCR-DeNO_x. The main components of particles are ammonium bisulfate and a small amount of ammonium sulfate. The effects of SCR-DeNO_x system on emission characteristics of fine particles attribute to three aspects. Firstly, some SO₂ can be oxided to SO₃ during SCR process at the function of catalyst, and then SO₃ would react with NH₃ and H₂O to form ammonium sulfate and ammonium bisulfate particles, which is a reversible process simultaneous with the de NO_x reactions. Secondly, SO₃ can also react with the escaped NH₃ and H₂O to form ammonium sulfate and ammonium bisulfate particles after SCR system. Moreover, the free alkaline earth metal oxide in the flue gas(such as CaO) reacts with SO₃ and changes the physical properties of the fine particles.