This study investigates the decadal modulation of ENSO’s impact on intraseasonal summer precipitation over eastern China. During 1991–2024, early summer (June) following an El Ni?o event tends to see increased precipitation over the middle and lower reaches of the Yangtze River and regions to the south, whereas late summer (August) experiences a decrease in these same regions. This inverse-phase relationship underwent a significant decadal shift around the early 2000s. A comparison between the periods 1991–2003 and 2004–2024 reveals that in the latter period, the relationship between ENSO and precipitation in August of the following year in the middle and lower reaches of the Yangtze River and regions to the south weakened, making it less likely for widespread negative precipitation anomalies to occur in this region. By examining the concurrent changes in sea surface temperature (SST) and atmospheric circulation associated with ENSO events, this study finds that over the western North Pacific in August 2004–2024, weakened easterly anomalies lead to reduced latent heat flux from evaporation, resulting in a shift from negative to positive SST anomalies. Meanwhile, positive SST anomalies over the northern Indian Ocean are comparatively weaker. Consequently, the anomalous anticyclone over the western North Pacific is unable to persist and transitions into a cyclonic circulation, which is detrimental to precipitation deficits over the middle and lower reaches of the Yangtze River and adjacent southern regions. In terms of circulation configuration, at the 500 hPa level, the East Asia–Pacific/Pacific–Japan teleconnection pattern shifts from a “+-” to a “-+” pattern in August 2004–2024; at the 200 hPa level, the East Asian subtropical westerly jet weakens. Both changes are unfavorable for precipitation deficits over the middle and lower reaches of the Yangtze River and southern regions. This research clarifies the decadal modulation of ENSO’s influence on intraseasonal summer precipitation in eastern China, providing a theoretical basis for improving intraseasonal precipitation prediction.