Abstract: To explore the physiological adaptation mechanism of Alchornea trewioides to the arid environment of karst area, The growth, photosynthetic pigments and fluorescence parameters of one-year-old cutting seedlings of Alchornea trewioides under light drought stress (14d), moderate drought stress (28d), and severe drought stress (42d) and re-watering conditions were measured and analyzed. The results showed that : (1) With the prolongation of drought stress time, the relative water content of leaves decreased significantly, and the plant height and basal diameter increased slowly. After re-watering, the relative water content of leaves increased significantly, while the plant height and basal diameter did not change significantly. (2) With the extension of drought stress time, the contents of chlorophyll a (Chla), chlorophyll b (Chlb), chlorophyll [Chl (a+b) ] and Chla/b in the leaves of cutting seedlings increased first and then decreased, and were significantly higher than those in the control (CK) after different degrees of drought stress and rewatering. After rewatering under mild and moderate drought stress, the contents of chlorophyll a (Chla), chlorophyll b (Chlb), chlorophyll [Chl (a+b) ] and Chla/b in the leaves of cutting seedlings were higher than those of the control, and they could be restored to the control level after rewatering under severe drought stress. (3) With the aggravation of drought stress, the fluorescence parameter Fo of cutting seedlings increased significantly, Fm, Fv/Fo and Fv/Fm decreased gradually, the parameters ABS/RC, TRo/RC and ETo/RC representing the activity of unit reaction center increased significantly, the parameters Ψpo, Ψo and ΨEo representing the electron transport activity of receptor side decreased, and the parameter DIo/RC representing heat dissipation increased sharply. After rewatering under different drought stress, Fo, ABS/RC, TRo/RC, ETo/RC, DIo/RC, ΨDo were lower than those under drought stress, Fm, Fv/Fo, Fv/Fm, Ψpo, Ψo and ΨEo were moderately restored, Fm could return to CK level, while Ψo and ΨEo did not return to CK level. It can be seen that the growth of Alchornea trewioides seedlings was less affected by mild and moderate drought stress, but more affected by severe drought stress. The content and ratio of photosynthetic pigments in leaves increased significantly after drought stress. The PSII reaction center was inactivated, the electron transfer was blocked, and the heat dissipation increased under severe drought stress, but it could still be restored to a certain extent when the water environment conditions were improved. Therefore, the Alchornea trewioides has strong drought resistance and high post-drought recovery ability, and can be used as a pioneer species in the process of vegetation restoration in Karst areas.