This study systematically investigates the impacts of three types of compound events involving El Ni?o-Southern Oscillation (ENSO) and South China Sea (SCS) Sea Surface Temperature (SST) anomalies on summer precipitation over the middle and lower reaches of the Yangtze River (MLY), and further elucidates the potential physical mechanisms by combining analyses of large-scale circulation patterns with the horizontal Rossby Wave Ray Flux (Li-Yang WRF) diagnostics. The results show that, in the summer following a strong El Ni?o event, SST anomalies in the Ni?o 3.4 region can excite a westward propagating Rossby wave train along the tropics, which first affects the SCS and induces the western North Pacific anomalous anticyclone (WNPAC). The WNPAC further warms the SCS central basin by driving the anomalous Ekman downwelling, thereby triggering another Rossby wave train that propagates northward and eastward. After converging over the northwestern Pacific, the two wave trains jointly modulate the East Asian circulation, leading to the formation of a cyclonic shear zone over the MLY, the Korean Peninsula, and areas near Japan. This circulation configuration enhances moisture convergence and upward motion, ultimately increasing summer precipitation over the region. Under different SST anomalous states, the convergence locations of the two wave trains in the northwestern Pacific vary: The effect of La Ni?a on anomalous circulation in the northwestern Pacific is opposite to that under El Ni?o conditions, and the convergence location of the two wave trains is further east and north. In general, the compound of warm SCS SST anomalies and El Ni?o tends to promote precipitation in eastern China, whereas the compound of cold SCS SST anomalies and La Ni?a is unfavorable for increased precipitation.