The analyses of conductivity-temperature-depth (CTD) temperature and salinity data of R/V (Research Vessel) “Shiyan 3” and R/V “Kexue 1” observed during the South China Sea Monsoon Experiment (SCSMEX) were made to describe the vertical structure and variation of sea temperature and salinity in the upper layer of northern South China Sea (SCS) before and after the summer monsoon onset.The stratification of the upper-layer water in northern SCS is more obvious in late April than July.Isotherm distribution at some sections forms a wave-shaped structure with obvious thermal trough and ridge.High surface temperature water was observed off northwest of Luzon Island in late April.The formation of a mixed layer in the surface layer northwest of Luzon Island is prominent in July.Water on the northern SCS continental shelf has obviously turned much fresher in July than April due to the effects of discharge from Zhujiang River and Guangdong coastal waters. The analyzes of conductivity-temperature-depth (CTD) temperature and salinity data of R / V (Research Vessel) “Shiyan 3 ” and R / V “Kexue 1 ” observed during the South China Sea Monsoon Experiment (SCSMEX) were made to describe the vertical structure and variation of sea temperature and salinity in the upper layer of northern South China Sea (SCS) before and after the summer monsoon onset. stratification of the upper-layer water in northern SCS is more obvious in late April than July.Isotherm distribution at some sections forms a wave-shaped structure with obvious thermal trough and ridge. High surface temperature water was observed off northwest of Luzon Island in late April. The formation of a mixed layer in the surface layer northwest of Luzon Island is prominent in July. Water on the northern SCS continental shelf has obviously turned much fresher in July than April due to the effects of discharge from Zhujiang River and Guangdong coastal waters.