By the methods of time-space series analysis and field monitoring, this paper studied the species diversity and its maintaining mechanism in Dongmen Eucalypt plantations of Guangxi. The results indicated that during the period of 1998 - 2003, continuous cropping of Eucalyptus plantation resulted in a reduction of species diversity. The plant species number of second generation was reduced by 54.43% in a 667 m2 plot, and by 50% in 18 sampling plots (each of 4 m2), and the species richness and Shannon-Wiener index of 6-year second generation forest decreased by 39.39% and 17.76%, respectively, compared with the first generation. The continuous cropping not only decreased the species diversity, but also changed the species composition and characteristics of the community. Life-form spectra of Eucalypt plantations varied with generation. The plant life-form consisted of microphanerophytes, liana phanerophytes, and nana-phanerophytes in the first generation, while the herbaceous-phanerophytes and chamaephytes occurred in the second generation. The k-strategy and r-strategy species accounted for 69.32% and 30.68% in the first generation, while 47.83% and 52.17% in the second generation, respectively. Therefore, continuous cropping of Eucalypt plantations was advantage for herb, therophytes and r-strategy species, but disadvantage for woody plants and k-strategy species. Seed band and vegetative body in soil were considered to play an important role in maintaining diversity. A new hypothesis, initial plant Diaspora, on the maintaining mechanism of species diversity in Eucalypt plantation was put forward.