Characteristics and mechanisms of the sub-seasonal evolution of the Northwest Pacific Anomalous Anticyclone (NWPAC) phenomenon during a decaying El Niño in summer were analyzed based on a variety of reanalysis data and observation data. The results showed that the NWPAC had a more northern shift and increased intensity in both July and August compared with June during a decaying El Niño in summer. NWPAC affects precipitation in East Asia during boreal summer by influencing the divergence of water vapor fluxes in the low-level troposphere. The abnormal rain belt in East Asia shifts northward along with the NWPAC. The anomalous precipitation decline and increase in incidental solar short-wave radiant flux under the ridge of the NWPAC can result in positive surface air temperature anomalies in the Indochina Peninsula, the Philippines, and southern China during a decaying El Niño in summer. With the northward shift of the NWPAC, the positive surface air temperature anomalies in East Asia also shift northwards. Local air-sea interaction processes may be one of the causes of NWPAC’s northern jump during a decaying summertime El Niño. Increased short-wave radiant fluxes and reduced upward sensible and latent heat fluxes on the southern side of the NWPAC can cause anomalous positive sea surface temperature anomalies (SSTAs), and positive SSTAs, in turn, could be detrimental to NWPAC’s maintenance by inducing convective instability. In contrast, weakened short-wave radiation fluxes and increased upward sensible and latent heat fluxes on the northern side of NWPAC can cause negative anomalous SSTAs, which favors the maintenance of NWPAC.