The alien weed Xanthium italicum (Italian cocklebur) severely decreases the Helianthus annuus (oil sunflower) seed yield; however, its mechanisms remain unknown. To explore the relative intensity of competition between the two plants and to test the hypothesis that interspecific competitive pressure from Italian cocklebur causes a reduction in the yield and quality of oil sunflower seeds, a replacement experiment using the two species was performed under simulated common water-supply conditions of sunflower farmlands, and differences in growth parameters, seed yield, and seed quality of oil sunflower between monoculture and interplanting conditions were compared. The results showed that compared with those under monoculture conditions, the seed yield (biomass), oil content, and 1000-seed weight of oil sunflower interplanted with Italian cocklebur decreased by 7.75%, 23.48%, and 13.86%, respectively, under low water supply. Moreover, these parameters were reduced by 19.85%, 24.36%, and 16.19%, respectively, under high water supply. The interspecific competitive intensity of X. italicum was significantly higher than that of oil sunflower under both water-supply conditions. Given the competitive inhibition from the exotic plant, the vegetative and reproductive growth of the native crop was significantly impaired, and its seed yield and oil content were significantly reduced.