During the period of the National Day of 2004, a continuous heavy air pollution event occurred. In order to have a good understanding of the causes of the event, a detailed analysis was made of the weather conditions during the time period by means of the air pollution index, conventional meteorological data, radiosonde data and AWS data. The results indicate that the pollution event was resulted from the combined influences of the large-scale synoptic background and local weather conditions such as weak wind, inverse temperature, high relative humidity, etc. , as well as local pollution sources.