Daily PM2.5 samples were collected at an urban site in Chengdu in the spring and autumn of 2012. Samples were subjected to chemical analysis for various chemical components including major water-soluble ions, organic carbon (OC), and element carbon (EC). The results showed that the average concentrations of PM2.5 in the spring and autumn were 101±64 μg m^-3 and 88±30 μg m^-3, which were 1.3 times and 1.2 times of the daily average of the ambient air quality standard, respectively. Based on K⁺, OC/EC (OC concentration/EC concentration), and K⁺/EC (K⁺ concentration/EC concentration), biomass burning episodes were identified. It was found that OC, EC, K⁺, and Cl^- in PM2.5 during biomass burning episodes were obviously higher than that during non-biomass burning periods; SO₄^2-, NH₄⁺, Ca²⁺, Mg²⁺, NO₃^-, and Na⁺ all increased in different levels during biomass burning episodes. OC during biomass burning episodes were 4.2 times and 1.8 times of that during non-biomass burning periods in the spring and autumn, respectively; and EC were 2.3 times and 2.3 times of that during the non-biomass burning periods. Concentrations of K⁺ and Cl^- during biomass burning episodes were three times higher than the average concentration in the spring, and 0.8 times and 0.9 times higher than the average concentration in the autumn.