Abstract: As the haze occurred frequently in Beijing, more and more attentions have been paid to air quality. A time series analysis method was used to describe the trend of air pollution index (API) in Beijing in 2001-2012, and to find out the correlation of API with precipitation, wind speed, temperature and humidity. There was a declining trend for API index in general, and a significant seasonal variation of API, which was the highest in the spring and winter, and the lowest in the summer; The API was higher in March and April, and lower in July and August. Among the primary pollutants, the proportion of SO₂ reduced as the proportion of inhalable particulate matter increased; the API index was negatively correlated with precipitation, and positively correlated with wind speed, temperature and humidity.