Abstract: Combining field investigation and laboratory measurements, the calculation and analysis of the relative abundance and niches, of a weed community of a flax field in the Lanzhou area were studied. There were 23 species from 11 families in the flax fields. The main families were Gramineae, Compositae, Chenopodiaceae, Amaranthaceae and Convolvulaceae, and the dominant species were Kochia scoparia, Setaria viridis, Chenopodium album, Sonchus brachyotus, Echinochloa crusgalli and Calystegia hederacea. Weeds appeared in mid-April, and there were two peaks of weeds, one in mid-May and one in-June. K. scoparia occupied the largest integrated niche breadth, being the most harmful to the flax, followed by S. viridis and C. album. K. scoparia and S. viridis shared the largest time niche overlap, and shared the maximum horizontal niche overlap with S. brachyotus, while S. brachyotus and C. album shared the biggest vertical niche overlap. All the above weed species had a similar use of resources. Using relative abundance and niche breadth can determine the dominant weed species, reflecting the degree of weed damage to the crop.