It has given rize to much controversy about the effects of forest on hydro logy The main issue is the effects of forest on rainfall, annual runoff and evapotranspiration. In order to know the reason causing the controversy, in this paper,a careful review on present research situation of forest hydrology is made according to analyzing a lot of research results in China and other countries. And then the effects of forest on hydrological factors (rainfall, streamflow, evapotranspiration, water aulity and groundwater level) in a watershed are mainly discussed, Besides, the effect of forested and watershed areas on forest hydrological elements including the research method is considered too.The effects of forest on hydrological factors in a watershed can be usually divided into three conditions which are positive, negative and zero effects The concrete result varies with specific conditions, so the effects of forest on hydrology can not be lumped together.The problems lie in the research on forest hydrology are as fallows. 1) to put undue emphasis on the effects of forest hydrology. and to isolatedly analyze each factors influencing forest hydrological process. 2) to study insufficently regulated water function owing to using comparable analysis method.These problems are also the reason for controversy because that the above mentioned issues can make researcher judge only according to the superfical phenomenon.