Abstract: During June-July 2024, an extreme abrupt drought-flood alternation event occurred over the Huang-Huai Valley (HHV), causing serious impacts on the social economy. In this paper, the characteristics and mechanisms and its corresponding prediction capability assessment of this extreme event over the HHV are investigated using station observation data, reanalysis data, hindcast and real-time forecast data. Results demonstrate the following points. (1) The precipitation over the HHV during June-July 2024 showed remarkable and distinct features. It involved a wide range of drought, an incredibly rapid transformation between drought and flood, and frequent rainstorm. The precipitation in June was less than 38.8%. However, the precipitation in July suddenly turned out to be 106.7% more than that in the same period. The abrupt short-cycle drought-flood alternation index over the HHV during June-July 2024 was the third highest of the drought-to-flood events since 1961. (2) A circulation pattern in June with low pressure in the east and high pressure in the west was situated in the mid-high latitudes over Eurasia, accompanied by stronger Ural Blocking High and more frequency of Cold Vortex over Northeast China. The water vapor passages in HHV controlled by dry flow coming from southwest are mainly located in Eurasia, which makes against the precipitation occurring in this region. (3) In July, the 500 hPa in the mid-high latitudes over Eurasia showed stronger the meridional flow. The HHV was located in the northwest of the western Pacific subtropical high (WPSH) with northward shift of the WPSH. Meanwhile, the upper-level and low-level circulation systems strengthened and shifted significantly northward corresponding with the WPSH. The HHV was located in a strong upper-level divergent area between the southern of the upper-level westerly jet and the northern of the ridge line of the South Asian high, further strengthening the development of the upward motion. The southwest monsoon transport enhanced and advanced northward, causing significant changes in the water vapor channel and the water vapor source. The HHV was located in a distinct water vapor flux convergence area. The upper-level and lower-level circulations configuration provided favorable thermodynamic and moisture conditions for the excessive precipitation in July. The circulation of drought-flood abrupt alternation in the HHV in 2024 showed the characteristics of typical years. The Boreal Summer Intraseasonal Oscillation (BSISO) is a key tropical low-frequency driver of the abrupt drought-flood alternation event during June-July 2024 , and the 30-90-day signal favors the northward shift of the WPSH. (4) The evaluation of the subseasonal prediction capability of the Western Pacific Subtropical High (WPSH) from June to July in the recent 10 years of model shows that the CMA-CPSv3 model (China Meteorological Administration Climate Prediction System version 3) has the best forecast skill for the ridge line of the WPSH, followed by the area and intensity, while the prediction of the westward extension ridge point still has much room for improvement. Further evaluation reveals that the prediction capability of the WPSH ridge line for drought-flood abrupt alternation event years is generally lower than the normal years with leadtime of 1 to 3 pentads, indicating that the model has limited prediction capability for the rain-band progression.