The changing characteristics of species diversity of Caragana microphylla pasture during the process of sandy desertification in Keerqin Sandy Land were studied. The invasion and disappearance of plant species, the change of dominance and life form structure, and niche breadth of dominant populations were also analyzed, The species diversity indexes analysis showed that desertification was a process of bio-diversity loss. And most of original species were wiped out in this process especially from fixed sand dune to semi-fixed sand dune stage. Change in life-form spectra indicated that Hermicryptophytes were severe affected by desertification. The drift of niche breadths of 17 main plant populations in the process of desertification was measured using the formulas described by Levins. The results showed that the niche breadths of most original species decreased and those of psammophytic species increased accordingly in the process of desertification.