Abstract: During the course of dehydration of Ginkgo biloba leaves, net photosynthetic rate(Pn), which has a significant correlation with stomatal conductance(Gs) (R2=0.816),went down sharply with the rapid decline of Gs until water saturation deficiency (WSD) up to 25%, whereas actual PSⅡ efficiency (ΦPSⅡ) only went down slightly. However, with further increase of WSD above 25%, Pn and Gs both decreased slowly and slightly and the correlation between them was insignificant (R2=0.152),while ΦPSⅡ descended rapidly and remarkably. Based on the analysis of such concerned parameters as stomatal limitation(Ls), inter cellular CO2 concentration (Ci) and ΦPSⅡ etc., the conclusion could be drawed that the main factor that led to drop of Pn lied in decline of Gs until WSD up to 25% but it lied in decline of photosynthetic activeness in leaves when WSD was above 25%. Under the long term drought stress in soil, Pn, Gs and water use efficiency (WUE) in Ginkgo leaves went down remarkably. Regardless of Gs went down significantly under drought stress in soil, the decline of Pn did not lie in it but in the decline of photosynthetic activeness in leaves. Also, drought stress in soil led to more severe decline of Pn in midday in Ginkgo leaves.