To characterize the chemical composition of PM2.5 and the formation of pollution during a heavy pollution episode in the winter in a typical logistics city, PM2.5 samples were collected from December 2016 to January 2017 at six sampling sites, and the water-soluble ions, elements, and carbon components were determined. The results showed that the average concentration of PM2.5 was (145.2±87.8) µg·m-3 during the whole sampling period, of which 82% of daily average concentrations were above class Ⅱ of the national standards. The average concentrations of PM2.5 during the two heavy pollution episodes were (187.3±79.8) and (205.3±92.0) µg·m-3, which were 5.4 and 5.9 times, respectively, as high as class Ⅱ of the national standard. The results of the chemical composition of the PM2.5 showed that secondary water-soluble inorganic ions (SO42-, NO3-, and NH4+) were the main components of PM2.5 in winter (51.2% of PM2.5 mass concentration), followed by organic matter (OM, 23.8%), and mineral dust (12.7%). Combined with the change trend and accumulation rate of chemical components during the pollution episode, we discovered that the increasing of SNA and OM led to PM2.5 pollution in the first episode, while the growth of SNA caused the second pollution episode. This was further verified by the synchronous change of SOR, NOR, and the OC/EC ratio. PMF analysis indicated that mixed sources of secondary particulate matter and biomass combustion (50.0%), coal combustion (16.8%), vehicles (12.9%), fugitive dust (10.0%), industry (5.3%) and soil dust (5.0%), were the main sources of PM2.5 of Linyi city in the winter. Compared with the average concentration over the whole sampling period, the contribution of secondary particles during the two pollution episodes was significantly increased. This indicates that the formation and accumulation of secondary particulate matter under static and humid meteorological conditions were the main influencing factors during the heavy pollution episodes.