Abstract: According to the oceanographic surveys on zooplankton in Nansha area before and after the onset of Southwest Monsoon(OSM) during March~April and June~July in 2013, we investigated the species composition, abundance, ecological groups, dominant species and diversity of the pelagic copepods. A total of 187 copepod species were identified, including 127 macro-copepod species and 152 meso-copepod species. The fauna of copepods belonged to the typical tropical ocean region with four ecological groups, such as oceanic species, euryhaline species, neritic species and deep-water species. The dominant species of the macro-copepods changed unobtrusively and those of the meso-copepods changed significantly with OSM transformation. The average macro-copepods abundance was 4.87 ind · m^-3, about a quarter of the average meso-copepods abundance of 19.2 ind · m^-3. The copepod diversity was rich or very rich. The horizontal distributions of the copepod community characteristics changed along with the surface currents and water masses by OSM transformation, and macro-copepods were more sensitive to the vertical movement of the water masses but duller to the change of the surface currents than meso-copepods.