As the eastern tributary of the Pearl River in Guangdong Province, southern China, East River is a major source of water supply for Hong Kong and Shenzhen. Besides their important role in nutrient cycling and transformation in the river ecosystem, fish are also considered as good indicators of aquatic ecosystem health. For the purposes of assessment and management of East River it is necessary to look into the status of fish spawning ground, especially with cascade dams' duress. From March to October, 2010, we studied taxonomic composition and daily variation of fish larvae abundance from a survey in the Guzhu section of East River over a period of 8 months, and collected 26 species (or genera). The fish larval samples were numerically dominated by four species, Gobies (Gobiidae) 63.0%, Barbel Chub (Squaliobarus curriculus) 16.8%, Tilapia (Tilapia mossambica) 9.7%, and Mandarin (Siniperca kneri) 2.0%. These four species accounted for 91.5% of total numbers. During the sampling period, the total fish larvae abundance was estimated as 1.14 billion. When compared with historical records from the 1950s' and the currently condition of fish larvae in the West River, there are obvious degradation in the fish larvae community in East River, which means no migratory species and irregular response to the hydrological variation. It comes to the conclusion that the cascade dams blocked the fish migration and caused the disappearing of some spawning grounds. Our results clearly show that Tilapia larvae outbreak in the East River, which is very dangerous to the ecology safety. We also conclude that construction of cascade dams makes the East River more habitable for Tilapia. For the purpose of protecting ecosystem, we recommend that more Silver Carp (Hypophthalmichthys molitrix), Bighead Carp (Aristichthys nobilis) and Black Guangdong Bream (Megalobrama terminalis) be released to the East River and possibly constructions of fish passage be considered.