Thermohaline staircase is a step-like stratification phenomenon of temperature and salinity of sea water increasing with depth, which greatly affect the material and energy exchange in the ocean. Previous studies have found extensive occurrence of thermohaline staircases in the Caribbean Sea, while research on its evolution is still in infancy. Contact measurements can carefully characterize the vertical structure of the temperature and the salinity. However, due to the sparse distribution of stations, the lateral resolution is limited. Seismic oceanographic methods with a high lateral resolution allow a better study of the spatial distribution of the thermohaline staircases and their continuity. A large number of the thermohaline staircase regions are found at depths of 400 m ~ 510 m in the eastern Caribbean Sea based on some seismic lines from the EW9501 cruise. We apply Particle Image Velocimetry (PIV) technology to consecutive pre-stack migration seismic sections and obtain a series of continuously varying flow velocity fields. The fine-structure changes of the thermohaline staircases are manifested in these seismic images as fracture, disappearance, and restoration of the reflection events. Changes in the internal fine-structure of the thermohaline staircases are related with internal waves, and changes at the edges of the thermohaline staircase region are associated with subsurface eddies. The study of changes in the fine structure of the thermohaline staircases is conducive to a deeper understanding of the interactions between the thermohaline staircases, internal waves and eddies.