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To study the seasonal pollution characteristics and sources of water-soluble inorganic ions in atmospheric PM 2.5 in Suqian City, 171 samples were collected at three monitoring points, which were in the water vapor channel, from May 2017 to January 2018. The mass concentrations of PM 2.5 and nine water-soluble inorganic ions were analyzed. The results showed that the annual average concentration of water-soluble inorganic ions in PM 2.5 in Suqian City was (44.08±34.61) μg ·m -3 , accounting for 41.8% of PM 2.5 . The concentrations of these species were in the order of ρ (NO 3 - ) > ρ (SO 4 2- ) > ρ (NH 4 + ) > ρ (ρl - ) > ρ (Na + ) > ρ (Ca 2+ ) > ρ (K + ) > ρ (F - ) > ρ (Mg 2+ ); NO 3 - , SO 4 2- , and NH 4 + accounted for 75.6% of the total water-soluble ions. The annual average ratio of ρ (NO 3 - ) to ρ (SO 4 2- ) was 1.53±0.88, indicating that mobile sources contributed more to PM 2.5 pollution. Based on the correlation analysis of NH 4 + and SO 4 2- , NO 3 - may exist in the form of (NH 4 ) 2 SO 4 , NH 4 HSO 4 , or NH 4 NO 3 . According to the principal component analysis, secondary transformation, industrial pollution, biomass burning, and dust were the major sources of water-soluble inorganic ions. PM 2.5 concentrations were positively related to relative humidity in winter. Water vapor transmission is more likely to promote PM 2.5 accumulation in winter.