Root border cells (RBC) are cells surrounding the root apex. They are functionally different from the apex and are considered to play a role in the protection of the root tip from biotic and abiotic stresses. In order to explore the allelopathic mechanism of an alien invasive plant in China, Amaranthus retroflexus L （redroot pigweed）, allelopathic effects of the aqueous extracts from different development stages of this species on root border cells of cucumber were studied by aeroponic culture. The results showed that the aqueous extracts of A. retroflexus could stimulate the activity of pectin methylesterase of the root cap of cucumber, but the stimulating effect gradually lowered down along with the increasing concentration of the aqueous extracts, and the viability of root border cells decreased as the concentration of aqueous extracts increased. There were no significant differences among different organs of A. retroflexus at the same developmental stage, while significant differences were found in the A. retroflexus at different developmental stages and the order of strength of allelopathy at different developmental stages was seedling stage>flower\|bud stage>ripeness stage. These results suggested that allelopathy should be one of the reasons that A. retroflexus can compete with other species in the community and occupy niche in early developmental stages.