Abstract:Flaveria bidentis (L.) Kuntze was a kind of aggressive alien plant which had invaded farmland in Hebei Province, China. To determine the economic infestation level and economic threshold of F.bidentis in the cotton fields and its critical period of competition, additive series experiment and curve fit were adopted to study the changes of yield characters of cotton under different densities of F.bidentis in the fields. The results showed that with the interference of F.bidentis, the leaf area index, light penetration rate, water use efficiency, utilization rate of nitrogen and output of cotton decreased gradually as the weed density increased. The leaf area index, light penetration rate, water use efficiency and utilization rate of nitrogen of cotton was significantly lower than those of control when the density of F.bidentis was 5 plants per m2 and the cotton yield was significantly lower than that of control when the density of F.bidentis was 2 plants per m2. The logarithmic regression model was relatively better in describing the relationship between F.bidentis density and yield loss of cotton (y=22.79 lnx+9.0277, R=0.9759). The economic infestation level for F.bidentis in cotton fields was between the range of 0.66% and 3.06%, the economic threshold was 0.69-0.77 plants per m2, the critical period of competition was 5.15-83.13 d after cotton sowing.