Abstract: The western Pacific subtropical high (WPSH), as a pivotal atmospheric circulation system affecting summer precipitation distribution in China, exhibited unprecedented spatial expansion in June 2025. This anomalous development resulted in the flood season onset in the Haihe River Basin (HRB) occurring approximately 13 days earlier than the climatological mean, accompanied by multiple heavy precipitation events. This study systematically investigates the underlying mechanisms driving the anomalous northward displacement of the WPSH in 2025 and its climatic impacts on the HRB, utilizing integrated ERA5 reanalysis data (1950-2025) and in-situ precipitation observations from China Meteorological Administration stations. The results reveal a persistent increasing trend (0.02/a) in the WPSH area index, which attained a historical maximum of 0.32 in 2025 representing an 80% deviation above the climatological mean. Concurrently, the intensity index has maintained elevated values since 2015, peaking at 6.76 in 2025. Diagnostic analyses demonstrate that the primary driver of this anomalous northward shift was the pronounced sea surface temperature anomaly within 25°-50°N latitudes. Through thermodynamical forcing mechanisms, this oceanic anomaly induced approximately 10° latitudinal northward displacement of t ascending motion in the mid-latitude western Pacific, consequently leading to markedly weakened ascending motions between 30°–40°N and triggering northward migration of the subtropical descending zone. These circulation anomalies enhanced water vapor transport by 14% relative to climatological norms across the HRB, directly contributing to four documented heavy precipitation events.