Huai'an and surrounding area experienced a serious fog-haze event during June 9—11, 2012.The concentrations of PM_2.5, NO_x, CO, SO₂ increased remarkably during this period.Based on the air pollution monitor data and the synoptic observation information, the air pollution characteristics, influence factors and the sources were analyzed.The results showed that the SO₂ concentration was always lower than 0.049 mg/m³, NO_x, PM_2.5, CO concentration was inversely related with visibility and had two peaks when the visibility was lower than 1 000 m in the whole process.During the fog-haze progress, inversion layers maintained at the low-level, whose bottom was at 975 hPa and the top near 925 hPa.The stable structure restrained the atmospheric turbulences, vertical exchanges of water vapor as well as vertical diffusion of pollutants, and thus created good thermal conditions for the maintenance of the fog-haze process.There remained easterly winds during the whole progress and the visibility changed with the wind speed synchronically.By tracking the air mass from the simulations of the HYSPLIY4.8 model, we found that this fog-haze process over the lower reaches of Huaihe River was mainly caused by the weather condition and crop residue burning.