The modulation of MJO ( Madden-Julian Oscillation) on the summer precipitation and its effect on sea surface salinity in the South China Sea is investigated based on the outgoing longwave radiation, precipitation, atmospheric reanalysis data and H YCOM salinity， with the composite analysis，significance test and correlation analysis. The MJO has significant modulation effects on the summer precipitation in the South China Sea, which exhibits strong intraseasonal variation with period of 45 days. The intraseasonal signal of precipitation shows significant changes in the windward slope including the northern Gulf of Thailand, the west of Luzon Island and the southwestern Taiwan Island, while weak in the leeward slope of the Annam mountains near Vietnam, and moves northeastward with the MJO signal. The analysis of dynamic factors suggests that the impact of the convergence in the lower troposphere and the convection in the middle troposphere is suppressed (enhanced) in the region of inactive (active) MJO convection, leading to low (high) precipitation. The influence between wind and anticyclonic ( cyclonic) circulation followed by inactive ( active) MJO convection shows that weak ( strong) wind maintain lower ( high) precipitation in the windward slope. The MJO-induced anomalous precipitation also influences the salinity of the South China Sea, causing strong intraseasonal variation in salinity with period is 47 days. The salinity anomaly signal moves northeastward with the precipitation anomaly.