Enclosure is one effective management method of native grassland, which is benefit for the improvement of grassland productivity and restoration of degraded grassland. We studied the species diversity, vegetation composition, plant species niche characteristics and grassland succession degree of grazing and enclosure grasslands in an alpine meadow on the QinghaiTibet Plateau, and analyzed the plant interspecific relationship and the direction of succession of alpine meadow. The results showed that: (1) the enclosure significantly decreased number of species，and α and β diversity indexes of grassland community, but significantly increased the number of species of therophyte and hemicryptophyte, and aboveground biomass. (2) For the aboveground biomass ratio, grazing grassland dominated by Cyperaceae(59.7%)and Gramineae(23.9%), while the enclosure grassland dominated by Gramineae(85.0%). For the dominant species, grazing grassland was Kobresia humilis and K. capillifolia, while for the enclosure grassland, it was Elymus nutans and Poa sp. (3) The pattern of importance value of plant species accorded with their respective niche breadth values. (4) The interspecific competition of grazing and enclosure grassland mainly exhibited between the species of different families or genera. The enclosure increased their overall niche overlap values and interspecific competition. (5) The succession degree of plant community was enclosure grassland > grazing grassland, and the enclosure community was in a relative stable state. The research indicated that the enclosure management can encourage the alpine meadow succession from sedges/forb community to grass/forb community.