During January 2013, Anhui Province experienced frequent and severe haze, which was characterized as widespread and long-lasting with very low visibility. The potential causes of this long-lasting haze are discussed by using the routine daily ground level observational data of three representative stations and high space-resolution sounding data recorded at two stations in January of the most recent five years, together with trajectory-cluster-statistics analysis. The statistics show that low wind speed and high humidity alone cannot explain the low visibility and abundant haze, which reached historical levels in January 2013. The more frequent and deeper ground inversion, and thus, higher stability of stratification occurring at that time can partly explain these situations. Although the back-trajectories of the air mass at the height of 1 km cannot explain the severe haze in Anhui Province, those at the height of 100 m can sufficiently explain these situations. At the height of 100 m, the northeasterly trajectories corresponded to the lowest average visibility at all three stations. In January 2013, the frequency of trajectories corresponding to the lowest visibility group, or together with the second-lowest group, was the highest among the same period of the most recent five years. Therefore, the main reasons of the low visibility and high haze frequency in Anhui Province in January 2013 were more stable stratification and more controlling air masses from northeast.