Abstract: To investigate the climate background for the extreme rainfall anomaly in 2021 over the mid-lower reaches of the Yellow river, this study analyzes the leading mode of autumn rainfall over the mid-lower reaches of the Yellow river during 1951—2021 and its relationship with the extreme rainfall anomaly in 2021. Rainfall data collected at 160 stations in China and NCEP/NCAR atmospheric circulation reanalysis as well as NOAA sea surface temperature (SST) reanalysis are used. The result of Empirical Orthogonal Function analysis reveals a consistent autumn rainfall pattern from the southeast of Gansu province to the west of Shandong province, which covers the mid-lower reaches of the Yellow river. This pattern is regarded as the leading mode of autumn rainfall over the mid-lower reaches of the Yellow river. The time coefficient in 2021 is the maximum since 1951, consistent with the extreme precipitation in 2021 in the region. The extreme event in 2021 is a typical example corresponding to the leading mode. This study uses the time series of Autumn Rainfall over the Yellow river (ARYR) to represent the variability of this leading mode. Analysis reveals that the interannual and interdecadal variations of this mode are affected by El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) with more rainfall during La Niña phase and negative PDO phase. Further, the leading rainfall mode over the mid-lower reaches of the Yellow river is closely related to SST anomalies in the mid-latitude North Pacific with more rainfall under higher SST. The mid-latitude North Pacific SST (MNPSST) index is calculated over the key region in the North Pacific, where the correlation is the most significant. The MNPSST index is the highest in 2021 since 1951. When the MNPSST index is high, the regressed high-level (low-level) anticyclone (cyclonic) shear occurs over the mid-lower reaches of the Yellow river, and strong upward motions develop over the mid-lower reaches of the Yellow river and the Marine Continent (MC) region. In the autumn of 2021, there are strong upward motions in the northern South China Sea, and anomalous easterly winds over the northern Pacific are obviously stronger. As a result, water vapor flux anomalies could split to southern and eastern branches, reaching the mid-lower reaches of the Yellow river basin. The leading mode of autumn rainfall over the mid-lower reaches of the Yellow river is strongly related to the intensity of SST anomaly in the North Pacific. Positive SST anomalies in the mid-latitude North Pacific are one of the most important factors affecting extreme rainfall over the mid-lower reaches of the Yellow river in autumn 2021.