Abstract: Using the traditional drought-flood abrupt alternation index (DFAI), it was difficult to classify abrupt alternations which leads to the misjudgment and omission for certain alternations. This study proposed an improved standardized drought-wetness abrupt alteration index (SDWAI), by clarifying the definition of drought-wetness abrupt alteration. Simulation and case study showed that the range of DFAI was large. The larger the weight parameter of DFAI was, the smaller the misjudgment rate of abrupt alternation event was but the larger the omission rate was, implying that misjudgment and omission of abrupt alternation events were unavoidable. The misjudgment generally occurred when a normal event turned into a severer flood or a severer flood turned into a normal for the adjacent flows. The omission normally occurred when adjacent drought and flood were light. Due to small differences between the ranges of SDWAI and drought-wetness index, the classified levels and their thresholds of drought-wetness abrupt alternations were consistent with those of drought-wetness assessment. The misjudgment of an abrupt event was avoided by recognizing in advance that the adjacent events were drought and wetness events, respectively. The omission of an abrupt event was avoided by the reconstructed weight parameter. In the Dongjiang River basin, which is the source of water supply to the Guangdong-Hong Kong-Macao Greater Bay Area, 33.8% the abrupt alternations from drought to flood mainly occurred in April and May at the beginning of the former flood season, and 18.5% of that from flood to drought mainly occurred in October at the end of the latter flood season. In addition, there were twelve years for the coexistence of the abrupt alternations from drought to flood and from flood to drought.