The air pollution index (API) and meteorological parameters in four cities (Harbin, Changchun, Shenyang, and Dalian) in northeastern China were analyzed in 2001-12, to study the characterization of the API and its influential factors. According to the monitoring data, air pollution is a significant problem in northeastern China, with all four industrial cities heavily polluted, especially Shenyang. During the study period, the API in the cities was down slightly. Clear interannual, seasonal, and monthly variations of air pollution were determined, which indicated that air quality was poorest in winter (especially November and December), but improved in summer (especially July and August). Air quality level varied in different weather conditions. Water vapor pressure was the most influential meteorological factor with regard to the API, followed by air temperature and surface pressure. Wind direction was found to be an important influential factor with regard to air pollution, because air flow from different directions has an impact on the accumulating or cleaning process of pollutants. However, the dominant meteorological factors influencing air pollution varied in each of the cities in terms of season, time scale, and level of air pollution. Our results highlight the significant impact of synoptic weather on API in northeastern China.