Abstract: Based on the data of four seasonal oceanographic censuses in 1997～2000 in the East China Sea (23°30′～33°N,118°30′～128°E),this paper discussed the relationships between the ecological characters of natural Undinula vulgaris population and its environment in the Sea,with the data of 1979～1980 compared.The results showed that there was a very clear seasonal change of the dominance,abundance,and occurrence frequency of U.vulgaris,i.e.,autumn (0.09,556 ind·100 m^-3,93.69％)>summer (0.03,86 ind·100 m^-3,68.49％)>winter (0.02,42 ind·100 m^-3,53.85％)>spring (0.01,16 ind·100 m^-3,34.35％).As a dominant species of pelagic copepods in the Sea, U.vulgaris showed its predominance mainly in autumn.The percentage of U.vulgaris in the total abundance of pelagic copepods was 2.31% in spring,4.80% in summer,3.80% in winter,and 9.90% in autumn,while its occurrence frequency was the highest in autumn and the lowest in winter.Comparing the results of this study with the data of 1979～1980,the importance of U.vulgaris in pelagic copepods in the Sea was decreased in spring,summer and autumn,but increased in winter,which might be related with the global warming in winter.The main environmental factors affecting U.vulgaris abundance were the bottom water factors that closely related with the warm current in the ocean.Because of its sensibility to low temperature and coastal water mass, U.vulgaris could be used as a good indicator of warm current,and,as a good food for fishes,this species plays an important role in marine fishing in the middlesouth part of the East China Sea.