We investigated the response of the East Asian Summer Monsoon (EASM) to a weakened Atlantic Meridional Overturning Circulation (AMOC) and its mechanism in an enhanced freshwater input experiment (FW) by using a fully-coupled climate model. The response was a weakened EASM and the mechanisms can be explained as follows. The simulated weakened AMOC resulted in a drop in sea surface temperature (SST) in the North Atlantic (NA) and, correspondingly, an anomalous high sea level pressure (SLP) over the North American regions, which in turn increased the northeast surface winds across the equator in the eastern tropical Pacific (ETP). The anomalous northeast winds then induced further upwelling in the ETP and stronger air/sea heat exchange, therefore leading to an anomalous cooling of the eastern tropical sea surface. As a result, the climatologic Hadley Circulation (HC) was weakened due to an anomalous stronger sinking of air in the ETP north of the equator, whereas the Walker Circulation (WC) in the western tropical Pacific (WTP) north of the equator was strengthened with an eastward-shifted upwelling branch. This feature was in agreement with the anomalous convergent winds in the WTP, and led to a weakened EASM and less East Asian summer precipitation (EASP). Furthermore, comparison with previous freshwater experiments indicates that the strength of EASP could be influenced by the magnitude of the added freshwater.