Abstract: This paper utilizes observation data from the SCMREX (Southern China Monsoon Rainfall Experiment) and numerical sensitive simulation experiments to investigate the influence of moisture amount and convection development over the northern South China Sea on MCS (Mesoscale Convective System) of a heavy rainfall event in coastal South China on May 8, 2014.Intensive soundings and wind profiler data reveal that there existed a convergence region formed by the southwesterly and easterly jet in the Pearl River delta, which provided a favorable condition for the development of convection.When the initial relative humidity was increased or decreased in the offshore area, or latent heat released from the cumulus and microphysical processes were turned off, significant effects could be found on the intensity and movement of convection in the coastal area of Guangdong owing to the adjustment of temperature and wind fields.In particular, when increasing offshore initial humidity, prosperous sea convection modified the circulation in the entire simulation area, and suppressed the development of convection over the land.Moreover, if latent heat from cumulus and microphysical processes were turned off, the low-level jet could reach further north, and the convective system moved to the northeast in the later stage.These experiments indicate that the offshore initial moisture field and convection are indeed important for precipitation forecast in the coastal area.Therefore it is necessary to enhance offshore observation and data assimilation method in the future.