Using in-situ hydrographic observations from January to December 2017, 3D thermohaline structure and seasonal variation in the Zhanjiang Bay is investigated in this study. The results show that: (1) In 2017, the annual mean temperature is 23−27℃, the mean salinity is 19−27, the mean potential density is between 11−17 kg/m³, and the mean buoyancy frequency (N²) is about 7×10⁻⁵−5×10⁻³ s⁻² in the Zhanjiang Bay. The vertical structure and horizontal distribution of N² are similar to that of the temperature, while the distributions of potential density and salinity are similar. (2) The temperature has significant seasonality with the highest value in summer, followed by autumn, and the lowest in winter. The maximum temperature difference between winter and summer reaches 15℃, while the seasonal mean of the salinity varies slightly. The ebb and flood have less influence on temperature and salinity, comparing with their seasonal variations. The thermocline is the strongest in summer with the maximum gradient reaches 0.7℃/m at 10 m, whereas it shallows to 5 m in spring and fall, and the water well mixed in winter. The halocline is prominent in summer and fall, with the maximum gradient 1.1 m⁻¹. The seasonal variation of the thermohaline in the upper layer and lower layer are consistent. (3) For the horizontal distribution, the temperature decreases and the salinity increases from the bay head, the mid bay, the bank region, the shoal region to the bay mouth. The average temperature difference between the bay head and the bay mouth is 2.3℃, and the salinity difference is 2.7. The temperature-salinity (θ-S) diagram shows banding distributions with one end as the low-temperature and high-salinity water in the bay mouth, one end as the high-temperature and low-salinity water in the bay head, and the other water masses are between them. Different bands are shown in different seasons.