Abstract: There were 19 species of weeds, belonging to 8 families, in alfalfa fields at Ordos, which was investigated by the method of Thomas invented W and visual observation with random sampling of multilateral plots. The resultes indcated that the most harmful weeds were mainly from the family of Gramineae and Chenopodiaceae, followed by Amaranthaceae, Compositae and Boraginaceae. In addition, the main weed communities after the first harvest of alfalfa were Salsola collina+Chloris virgata+Amaranthus retrofexus+Echinochloa crusgalli+Lithospermum arvense+Chenopodium glaucum+Xanthium sibiricum+Setaria viridis. Among them, the hazard level of first four weeds was highest, showing a higher hazard degree on alfalfa than other weeds. However, after the second harvest, C.virgata+S.collina+L.arvense+C.glaucum+A.retrofexus+S.viridis were investigated as the dominant weed communities in alfalfa field. Among them, the hazard level was higher among first three weeds. Moreover, the index influence of weeds based on the method of fuzzy synthetic evaluation was established to reflect comprehensively the influence degree of different weed components after the first harvest of alfalfa, that is, S.collinaE.crusgalliC.virgataA.retrofexusC.glaucumC.glaucumS.viridisX.sibiricum. Five indicators, namely, relative abundance, plant height, hazard degree, weed quality and impacts of weeds hair and thorns on palatability should be integrated to comprehensively and systematically reflect the relative harmfulness of various weed components on alfalfa.