Smoke-water (SW) had been reported to improve the growth of Isatis indigotica, a Chinese medicinal plant. However, there were very few reports on the mechanism of smoke-water improving plant growth. In this study the effects of smoke-water on the photosynthetic characteristics of I. indigotica seedlings were investigated for the purpose of understanding the mechanism behind this improved plant growth. The results showed that net photosynthetic rate (Pn) was increased by smoke-water, reaching a maximum on 15, 5 and 15 d after treatment with smoke-water at dilutions of 1:500, 1:1000 and 1:2000 respectively. Transpiration rate (Tr) and stomatal conductance (Gs) both showed similar trends toPn, however, intercellular CO2 concentration (Ci) was decreased with smoke-water treatment. The Fv/Fm was not significantly influenced by smoke-water treatment. The ?PSII was markedly promoted with the application of smoke-water (1:1000) compared with the control and the coefficient of photochemical quenching (qP) showed a similar trend to ?PSII. However the coefficient of non-photochemical quenching of chlorophyll (NPQ) was decreased with treatment of smoke-water. These findings indicate that smoke-water treatment induce an increase in photosynthesis and suggest the main factors leading to this might be the improved stomatal conductance and the enhanced level of the photochemical efficiency of PSII in leaves.