Abstract: In this article, the diurnal and day to day variation of solar total radiation, upward solar radiation, downwelling long-wave radiation, upwelling long-wave radiation and net radiation budget in South China Sea during May and July before and after South China sea monsoon onset have been analyzed, using the radiation observation data of "South China Sea Sea-Air Flux Measurements During 2000" project The analysis results show, before and after monsoon onset, nephsystem increasing and cloud layer thickening have remarkable effect on total solar radiation, upward solar radiation, downwelling long-wave radiation and upwelling long-wave radiation. During monsoon onset, the great wave caused by monsoon have much effect on upward solar radiation and reflectivity of sea surface and can cause the strength of upward solar radiation and the reflectivity of sea surface to increase rapidly.