The present study explores the effects of convection-SST interactions on the onset of the South China Sea Summer Monsoon simulated by the superparameterized Community Atmosphere Model (SPCAM). The SPCAM is a global multi-scale modeling framework that embeds a 2-D cloud-resolving model in each grid column to replace the conventional convective parameterization. Two experiments are performed: CTRL uses prescribed sea surface temperature climatology, and CPL is coupled to a slab ocean model (SOM). The bias of excessive seasonal mean precipitation over Asia during boreal summer in CTRL is reduced in CPL. In the South China Sea, the seasonal evolution of precipitation and 850 hPa winds is more realistic in the coupled simulation. During the pre-onset stage, the mean pattern of synoptic flow and precipitation, as well as the land-ocean diurnal cycle contrast is also improved in CPL. The coupling to SOM does not change the sensitivity of precipitation to column moisture in the SPCAM. The improvements in CPL can be partly attributed to the lower SST in response to air-sea interactions, and also partly to the suppression of heavy precipitation under high SST regime likely associated with a different atmospheric meridional circulation. Our current results demonstrated that the SPCAM coupled with SOM could be a potential tool to study the interactions among convection, SST, and large-scale atmospheric circulation from seasonal to sub-seasonal time scales.