Abstract: Floods are often prone to sudden occurrences in the short term, resulting in huge casualties and property losses. However, there is no consensus on the screening and analysis of sudden floods. Based on the assessment of the daily flooding status by the standardized antecedent precipitation index (SAPI), the abrupt flood index (AFI) is proposed to comprehensive reflection of the abrupt amount of water from the previous period (the same day) to the late period (the last 10 days) and the degree of flooding. At the same time, the definition and calculation of AFI threshold AFI_t are presented, considering that an abrupt flood event will occur later when AFI is greater than AFI_t . Taking the upper Hanjiang river basin as an example, the daily AFI index of the basin from 1972 to 2017 was calculated, and the date of the critical state was determined by AFI_t. Further analysis shows that the AFI index can better reflect the phenomenon of abrupt flood and AFI is helpful to identify the abrupt floods events and is benefit for the emergency management of the water resources system in the basin.