Abstract: The increased extreme precipitation in recent years has significantly affected human life and caused urban flooding. Therefore, forecasting extreme precipitation and identifying its precursor factors are of great importance. In this study, we analyze the spatiotemporal characteristics of the extreme precipitation event that occurred in northern China during September–October 2021, and examine the associated circulation background and influencing factors. Results show that the accumulated extreme precipitation exceeds 220 mm, making this event the strongest autumn extreme precipitation since 1961. Autumn precipitation in China is mainly affected by the ENSO (El Niño–Southern Oscillation). During this period, a developing La Niña phase induced an anomalous anticyclone that enhanced water vapor transport. However, the influence of ENSO was mainly confined to regions south of 39°N, whereas extreme precipitation north of 39°N was primarily associated with the anomalous activity of the MJO (Madden–Julian Oscillation). The MJO was anomalously active in phases 3–5, with an intensity 2–3 times its climatological mean. This enhanced intraseasonal-scale upward motion promoted water vapor convergence, resulting in the occurrence of extreme precipitation. Statistical analyses further show that the probability of extreme precipitation in northern China increases by more than 60% when the MJO is active in phase 4. These findings provide valuable insights for improving the forecasting and early warning of extreme precipitation events in northern China.