Abstract: Based on in-situ hydrographic observations in 36 stations in mid-May, 2025, the characteristics of temperature and salinity in the southwest sea area of Hainan Island were investigated in depth, and the related factors affecting its distribution were discussed. During the observation period, the seawater temperature in the study area was generally higher in the central basin and lower in the northwestern and southeastern zones. Notably, Sanya Bay showed pronounced cold anomalies in bottom layer. In horizontal distribution, the salinity was higher in the western zone and lower in the eastern zone. The low-salinity areas were concentrated in the region east of 108.8°E, and the minimum salinity occurred in Sanya Bay. In vertical distribution, thermocline and halocline were relatively distinct in most stations, and the thermocline was shallow, intensive and thin in general. The shallow nearshore stations showed strong vertical mixing and minimal changes in temperature and salinity with depth, with no thermocline and halocline. In addition, the thermohaline structure in the northwestern zone of the study area was closely related to the tidal mixing, where the upwelling caused by tidal fronts resulted in the decrease of seawater temperature. The intrusion of external seawater was the major reason for the occurrence of high temperature water in the central part of the study area. The characteristics of low temperature and low salinity in the Sanya Bay were mainly attributed to the combined effects of cold water inflow from the southeastern direction, coastal runoff, and precipitation. The inactive water exchange between Sanya Bay and the outer sea, plus the strong vertical stratification, cold water was confined to the bottom of the bay, making vertical mixing and horizontal diffusion hard.