Abstract: Many Studies have been carried out to deal with effects of floods on aquatic plant diversity of fresh water lakes. But no attention has been paid to understanding the effects of droughts on the diversity up to now. Changhu is the third largest lake (fresh water) in Hubei Province of China. It has many kinds of functions, such as store of flood water, irrigation, aquaculture, water transportation, and so on. Combinating Transect Sampling Method with Geographical Information System Technique (GIS) and Global Position System Technique (GPS), the authors studied the effects of a drought on aquatic plant diversity of the lake by comparing the aquatic plant diversity of the lake in 2000 (a drought year) with that in 1999 (a usual year). The function mechanisms of the drought on the aquatic plant diversity were also discussed by the authors. The main results are given as follows: (1) The drought had almost no effects on species diversity of aquatic plant, but increased the number of dominant species of the aquatic vegetation in the lake from 12 to 14. (2) The number of aquatic plant associations increased from 14 in 1999 to 18 in 2000. Species diversity indices of the associations by Shannon-Wiener Index and Simpson Index also remarkably increased under the drought condition. (3) The drought significantly enhanced the vegetation coverage from 45.15% in 1999 to 66.48% in 2000 and mean biomass per square meter in the whole lake from 1039.1g/m2 in 1999 to 1904.8g/m2 in 2000. (4) Under the drought condition in 2000, the emergent vegetation disappeared from the lake, but both the floating-leaved vegetation and submerged vegetation expanded greatly, from 3.71% and 41.32% in 1999 to 12.63% and 53.84% in 2000, respectively (the percentage here means the ratio of the floating-leaved vegetation area or submerged vegetation area to the whole lake area). (5) Both the growth and development of aquatic plants in the lake were accelerated by the drought. It was found that all the turion germination percentage of Potamogeton crispus, the shoot length of young plants of the species, and the ratio of yellow leaves of Trapa incisa to its total leaves in July 2000 were much greater than those during the same period in 1999. (6) It was concluded that the improvement of aquatic plant diversity in the drought year was mainly due to increasing of the light intensity which aquatic plants could get and increasing of the temperature in water body.