A largescale haze process happened on 12 to 13 December 2008 over Jiangxi Province is analyzed comprehensively based on the air pollution index, conventional observation, and radiosonde data. The results show that the joint influence of the southward moving cold air induced by the vertically turning horizontal trough and the dry warm tongue in the middlelevel layer is the background cause of the haze process. The cold air could not invade the highlevel layer above 700 hPa rapidly because of the restraining effect of the dry warm tongue, and then there appeared a “temperature inversion” structure, which facilitated the formation and maintenance of the largescale haze process. Affected jointly by the diurnal temperature difference and the cold air, with a relative low humidity, the weather changed frequently between haze and light fogs. A lower height of mixing layer made the pollutants move horizontally and spread along the mountains, and consequently turned the haze area into a eastwest pattern from a northsouth pattern.