Abstract: In freshwater ecosystems,higher trophic level group may influence lower trophic levels strongly,leading to a change of the whole enviroment,which is called top down effect.The top-down effects in freshwater ecosystems especially that of fish and their consequences were analized in the present paper,and it was suggested that the lack of high trophic level group which existed before can also make top-down effect.Top-down effect is the most important characteristic of freshwater ecosystem.While dealing with biodiversity conservation in freshwater ecosystem,much attention should be paid to conservation of high trophic level group and cautious to introduction of extra species.