Rapid development of China’s industrialization and urbanization in the past decades has highly decreased air quality. For instance, the Yangtze River Delta, a major economic area in China, is incurring strong haze pollution, yet precise pollution sources are unknown. Here, we hypothesized that sources of haze pollution might be the same in nearby cities within the region. To test this hypothesis, we studied sources in four major cities, Hefei, Hangzhou, Nanjing, and Shanghai, during the strong haze period from November 28 to December 10, 2013. This period was divided into four periods according to air PM_2.5 concentrations (PM: particulate matter): slight haze, moderate haze, heavy haze, and severe haze periods. Common pollution source areas were identified for the first time by backward trajectories and concentration weighted trajectory maps of PM_2.5. Results show that all cities contain air masses transported from the northwestern and northeastern regions. Emissions came mainly from northern and central China during the moderate haze period and from adjacent provinces during the severe haze period. During the heavy haze period, common sources were mainly located in the Anhui province, while during the severe haze period, common sources were mainly located in the northeastern part of the Anhui province and the western part of the Jiangsu province. Overall, our findings show that areas of pollution sources vary with the intensity of haze pollution. Our mapping method should thus provide more precise information to control air pollution at the regional scale.