Four treatments of water supply (serious drought-150mm, medium drought-250mm, light drought-350mm and plenty water supply-450mm) were adopted in this experiment, to study the effects of various water supply treatments on the growth and photosynthetic characters in Scutellaria baicalensis during the growing season. The results showed that under the water supply of 250mm and 350mm S. baicalensis grew better than other treatments with high biomass of shoot, root and the total, and the daily average of Pn is 12.76～13.24μmolCO₂•m^－2•s^-1. Lower biomass of shoot, root and the total were observed under the water supply of 150mm and 450mm. A two-peak curve in photosynthesis was presented with a midday depression in 350mm and 450mm water supply treatments. There was a significant positive correlation between Pn and Ci (p<0.05) under water supply of 250 mm and 350 mm. WUE was higher in 150mm treatment with a significant difference compared to the treatment of 450mm (p<0.05). Pn reduction in S. baicalensis originated from the co-effects of stomata factor and non- stomata factor. Stomata factor is dominant in the morning and in the afternoon, and non- stomata factor is dominant at noon. Appropriate dry condition may increase the Pn, root biomass and the content of root baicalin, which is for pharmaceutical use, in S. baicalensis, since it is a drought tolerant plant. These results suggested that the production of S. baicalensis in Jilin province should be arranged in the regions of precipitation ranging from 250 mm to 350 mm.