Corona discharging characteristics is of crucial significance to particle charging and migration procedures in Electrostatic Precipitator (ESP). In this article, the effect of SO₃ aerosol on discharging characteristics of ESP was investigated based on the results of experimental and numerical simulation method. The results showed that an increase in SO₃ concentration and relative humidity of flue gas lead to larger number concentration and particle size. Particle number concentration increased from 2.34×10⁷cm^-3 to 4.49×10⁷cm^-3 while SO₃ concentration increased from 17.5mg/m³ to 179.6mg/m³, which also resulted in an increase of median diameter from 0.08μm to 0.15μm. The corona current decreased by 78% compared with blank condition when SO₃ concentration was 179.6mg/m³, and higher flue gas humidity further deteriorated the corona suppression phenomenon. Meanwhile, the particle size and number concentration were important reason for the decrease of corona current. Particle with the size of near 0.1μm had large number concentration, which induced a high space charge density, suppressing the electric field and ion concentration around the electrode and then lowering the corona current.