From 25 to 27 August 2024, a Yellow River cyclone induced extreme rainfall stretching from the Beijing–Tianjin–Hebei region to Shandong Province, with numerous stations in Shandong breaking historical records for both hourly and daily precipitation. Despite temporal continuity and similar synoptic-scale forcing, the event unfolded in two distinct phases with contrasting precipitation characteristics: Phase I featured widespread, moderate-intensity rainfall across the extensive plains of the Beijing–Tianjin–Hebei region, whereas Phase II produced intense, highly localized extreme rainfall confined to the northwestern coastal area of Shandong. Leveraging ERA5 reanalysis data, high-density surface automatic weather station observations, and HYSPLIT moisture trajectory analyses, this study systematically compares the moisture transport features between the two phases and investigates their underlying mechanisms from both Eulerian and Lagrangian perspectives. The results show that: (1) Moisture during both phases originated primarily from the Beibu Gulf and the Taiwan Strait, with a significant increase in the number of moisture trajectories ascending from the Taiwan Strait and propagating northward along the eastern coastal waters during Phase II. (2) Differences in low-level circulation patterns and the intensity of dry air intrusion governed the contrasting moisture flux and convergence structures. In Phase I, a strong southwesterly low-level jet on the southeastern flank of the cyclone sustained persistent meridional moisture transport into the southern boundary of the rainband, coinciding with broad-scale moisture flux convergence and facilitating widespread precipitation. In Phase II, moisture influx shifted to the eastern edge of the rainband, where markedly enhanced low-level easterlies north of the cyclone rapidly advected warm, moist air from the Bohai Sea westward into the precipitation core. (3) Dry intrusion intensified over the cyclone’s northwestern flank during Phase II, driving upper-level dry, cold air to descend isentropically southward and downward into the cyclone’s northern sector. This suppressed moisture convergence over the western and northern regions, effectively confining low-level southerly moisture transport to a narrow zone east of the cyclone and establishing a localized, intense moisture convergence maximum—thereby creating the critical environment for the outbreak of extreme rainfall. This study demonstrates that, even under a common large-scale moisture source framework, synoptic-scale circulation adjustment—through the reconfiguration of moisture pathways and synergistic interaction with dry intrusion— is one of the reasons for abrupt transitions in precipitation characteristics, thereby providing new insights into the diverse modes of moisture organization in Huanghe River cyclone–related extreme rainfall.