Size-segregated samples (< 2.5, 2.5–5, 5–10, and 10–100 μm) and PM2.5 samples were collected to analyze the water–soluble inorganic ions (WSIs, including F−, Cl−, NO3−, SO42−, Na+, NH4+, K+, Mg2+, and Ca2+), through ion chromatography from January to October in 2017 in Jinzhong. The median concentration of the total WSIs in PM2.5 was 37 μg m−3, thereby accounting for 31% of the PM2.5, with the lowest level in spring and the highest in autumn. SO42−, NO3−, and NH4+ were the most abundant substances and were primarily on the fine particles (0–2.5 μm), whereas Ca2+, Mg2+, and F− were concentrated on the coarse particles (2.5–100 μm). The results of the correlation analysis led to the conclusions that (NH4)2SO4, NH4Cl and K2SO4 were the primary compounds on the fine particles, MgSO4 and CaSO4 were the major chemical forms of WSIs on the coarse particles, thus indicating that the formation mechanisms of these compounds were different; however, NH4NO3 and KNO3 were present in both the types of particles. The particles that were observed in Jinzhong were alkaline during the study period, and their acidity was negligible. The ratio analysis showed that the highest ratio of Cl−/K+ was found in winter in both fine and coarse particles; however, no obvious distinction has been made between Mg2+/Ca2+ during the four seasons. The NO3−/SO42− ratio in coarse particles was observed to be significantly higher than that in fine particles, particularly in summer, thus indicating that the heterogeneous reaction on particles plays a vital role in the formation of NO3- in coarse particles. The PCA analysis showed that the primary factors of WSIs, which were secondary formation, coal combustion, biomass burning, dust particles, and industrial emission. The coal combustion and biomass burning have been considered as the leading emission sources to be controlled for improving air quality in Jinzhong.