Abstract: Based on the cited hydrographical data from the East China and Yellow Sea, the blooming process and required conditions of Enteromorpha prolifera were studied. The results indicated that the frequency of red tides was decreased significantly in the Changjiang Estuary (CE) during recent years. In May, 2008, E. prolifera gradually developed in the CE, since there were abundant nutrients and favorable ecological environment such as optimum temperature (20-25 ℃) in the CE. Subsequently, the population was carried from the CE north-eastward to the southern Yellow Sea by the Changjiang Diluted Water (CDW). The flow of CDW slowed down in the southern Yellow Sea, and the population of E. prolifera aggregated there. Then small scale E. prolifera populations were developed in the southern Yellow Sea. Thereafter, the populations were dispersed and carried to central Yellow Sea due to the effect of sea surface current. In the Yellow Sea, the surface current runs northward, but in the process of movement, it was hindered by the Shandong Peninsula, forming E. prolifera aggregation further. Then current rip formed between the surface current and the Yellow Sea Coastal Water. Consequently, E. prolifera was bloomed in waters off Shandong Peninsula. Moreover, the outline of the Jiaozhou Bay is concave. All these were beneficial to the aggregation of E. prolifera in the bay. Therefore, E. prolifera was massively bloomed in Jiaozhou Bay. That’s why the E. prolifera was aggregated and bloomed in the waters off Qingdao in the year 2008.