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A severe haze event happened from 25 to 28 December 2005 in the downtown area of Chongqing is analyzed based on NCEP reanalysis L band radar, conventional observation, and radiosonde data, as well as the air quality data. The results show that the severe haze event was related to the certain synoptic background. The southwesterly airflow before the wide southern branch trough at the south side of the Tibetan Plateau was strong at 500 hPa, and Chongqing was controlled by the uniform pressure field at 850 hPa, resulting in the small pressure gradient force and horizontal velocity. The cold air activities affecting Chongqing were few and weak, which were beneficial to the formation and maintenance of the severe haze event. The small wind speed at the low level and the sustained existence of inversion layer at mid low level, as well as the low temperature, were the important conditions of the haze. Under the unsaturation circumstances, the appropriate increase of humidity was conductive to the strengthening of the haze. During the haze, the coagulation of moisture particles would aggravate the visibility.