Abstract: Based on a series of CTD data observed in Daya Bay and its adjacent waters during spring tide in July 2018, the distribution of temperature and salinity and their diurnal variation were characterized, and their possible causes were discussed. The results showed that the surface temperature distribution presented a trend of “high inside and low outside” from the top to the outer of Daya Bay, and the salinity distribution gradually increased from the inside to the outside. The 10 m layer also showed the characteristics of high temperature and low salinity in the inner bay and low temperature and high salinity in the outer bay. Vertical mixing in the bay is more uniform. Due to the overlap of the upper and lower water bodies, there is a strong thermocline and halocline outside the bay. The diurnal variation of temperature and salinity varied from place to place, and the difference is significant. The change is mainly due to the influences of shortwave radiation, warm drainage and tidal current.