Abstract: Based on the hydrographic data measured in January 2018, the characteristics of temperature and salinity in Daya Bay were analyzed in detail. The results show that the sea surface is generally characterized by higher temperature and lower salinity compared to the near bottom. Meanwhile, both in the sea surface and the near bottom layers, the sea water inside Daya Bay is characterized by higher temperature and lower salinity compared with that outside the bay. During the observational period, there existed a high temperature center on the west side of the bay, which should be affected by the warm water discharge from Daya Bay nuclear power plant. The difference in salinity is more obvious near the bottom. One low salinity center is located in the bay head area and the other lowsalinity center is located in the bay center, while the high salinity center is mainly located near the west sides of the bay mouth and the eastern Huidong coast. Solar radiation and tidal current are two important factors influencing the evolution of temperature and salinity in Daya Bay. The effect of solar radiation is mainly limited to the upper 3-4m water layer, and it is weaker on the nearbottom seawater. The heating causes significant diurnal variations of surface water both inside and near the mouth of the bay. The relations of tide to temperature and salinity show that the tide has a significant influence on the temperature and salinity inside the bay, but less influence outside the bay.