The characteristics and its possible mechanisms of winter flooding in Hunan in 2015 were investigated. The results show that: (1) The monthly mean precipitation in Hunan was 1.6 times more than the annual mean, which was recorded as the most for the corresponding period since 1961. Meanwhile, the precipitation amount was more than 250 mm in parts of southeastern Hunan, where flooding occurred. (2) Because the precipitation amount in November was above the normal precipitation, the atmospheric circulation anomaly was the most direct cause for the winter flooding in Hunan. The unusually positive precipitation anomalies in November was produced from the stronger location of Western Pacific Subtropical High (WPSH) and the larger western extension of its ridge, which easily transported water vapor from India and Pacific to East Asia and met the cold air down to southern China. (3) When the Pacific Decadal Oscillation (PDO) was in positive phase, the El Ni〖AKn~D〗o event was important external forcing condition for abnormal precipitation. The sea surface temperature of the central and eastern equatorial Pacific and Indian Ocean were anomalously above average, leading to the enhanced anomalous ascending motion over these oceans, while the anomalous descending motion over the Maritime Continent and its surrounding area occurred. These were favorable for the westward stretch of the WPSH, the anomalous descending motion over the low latitude East Asia, and the anomalous ascending motion over the midlatitude regions. The abnormal Hadley circulation caused strong convergence over the midlatitude, which led to continuous rain weather in these areas.