Abstract: Based on the observed data during four comprehensive investigations on ecological environment of Nansha area in2013, we investigated the seasonal variability in mixed layer depth ( MLD) in that area. Significant seasonal variability in MLD in that area was observed. In autumn, the MLD reached its maximum ( 65 m) , followed by winter ( 54 m) , summer ( 49 m) and spring ( 37 m) . These seasonal variabilities were closely related with variations inmonsoon, ocean current and sea surface heat flux. The MLD in the southeast area to the Indo-China Peninsula was deeper in spring; the MLD distribution was uniform in summer; the deepest MLD appeared in the central area in autumn; high value appeared in the northeastern area in winter. Ekman transport led to the effects of accumulation of water in the west Palawan Island, the south Nansha area and northwestKalimantan Island, as well as heat lost effect on north of 9Narea in winter, whichmight be the reason for regional difference of MLD. Moreover, the water temperature of mixed layer in that area was always over 27 ℃ ( more than 28 ℃ in summer) , and seasonal change of water temperature in the mixed layer was very little. When the regional difference of sea surface temperature ( SST) was huge, so was the space difference of MLD. Difference was found in relationship between MLD and SST between north and south in the surveyed area. Representative stations results reveal that MLD was contrary to the change of SST in the northern area, while in the southern area, the relationship between SST and MLD was not obvious. Since the southern area is near the equator, the change of solar radiation was little throughout the year which led to the little SST change. MLD rapidly deepened fromspring to winter, which was not obviously related with SST.