We analyzed the structure, distribution, and seasonal changes of planktonic copepod communities, as well as their relationships with environmental factors using data collected from 21 stations in the northwest coastal waters off Hainan Island between November 2008 and July 2009. A total of 44 species of planktonic copepods was identified, with 25, 23, 22, and 23 species found in autumn (November), winter (January), spring (April) and summer (July), respectively. The dominant species were mainly warm water species, such as Canthocalanus pauper, Subeucalanus subcrassus, Temora turbinate, Acartia spinicauda, Calanopia elliptica and Euchaeta concinna, etc. There were significant changes in copepod abundance among seasons; total copepod abundance was highest in winter with 409 ind./m³, followed by autumn with 144 ind./m³, spring with 55 ind./m³ and summer with 17 ind./m³, respectively. Distributional data showed that an area of high copepod density appeared in the southern portion of the study area in winter, while there was an area of high copepod density in the center of the survey area in autumn. Copepod abundance was higher in deeper waters, and lower in the coastal areas in spring and summer. Species diversity index was highest in summer and followed by spring, autumn and winter in turn. The evenness index of spring and summer copepod communities was higher than those of autumn and winter. The structure of these planktonic copepod communities reflects tropical and subtropical floristic characteristics. Species composition varied significantly with season, and copepod abundance was noticeably affected by water temperature and diatom cell abundance. We detected no obvious correlationship between copepod abundance and salinity.