Karst rocky desertification is a land degradation process,involving serious soil erosion,extensive exposure of basement rocks,drastic decrease in soil productivity,and appearance of a desert-like landscape.As it is a serious problem in Guizhou Province,studies were carried out on succession of the vegetations on lands under karst rocky desertification and its impact on chemical composition of runoffs from the lands in this region through vegetation surveys and analysis of soil and runoff samples.The results show that with development of karst rocky desertification,the soil was losing organic matter rapidly, becoming clayey in texture,decreasing in soil capillary porosity and in water-holding capacity in the surface or subsurface soil layers significantly during dry seasons,thus altering the law of water movement in karst ecological system.The analysis of chemical composition of surface runoffs show that in terms of concentration,anions and cations in the surface runoffs were in the order of HCO₃^->SO₄^2->Ca²⁺>Mg²⁺>K⁺、NO₃^-、Cl^->Na+>NH₄⁺>PO₄^3-.With karst rocky desertification going on, the output of PO₄^3- in the surface runoff increased significantly,and that of Ca²⁺ and NO₃^- followed,thus leading to decrease in soil fertility and affecting environmental quality of the recipient waterbodies.The variation of chemical composition of ground runoff was similar to that of the surface runoff,but the concentrations of HCO₃^-,Ca²⁺,and Mg²⁺ were higher than that in the surface runoff,respectively,while the concentrations of NH₄⁺ and K⁺ lower than that in the surface runoff.The concentrations of HCO₃^- and Mg²⁺ were reduced obviously with karst rocky desertification proceeding on,thus weakening karstification,while the concentrations of NH₄⁺ and NO₃^- were increased significantly,thus affecting the quality of ground-water,to some extent.