It has been demonstrated that Ipomoea batatas (L.) Lam (sweet potato) has greater soil nutrient absorption ability when grown in association with the invasive plant Mikania micrantha Kunth (mile-a-minute), but the competition interaction of the two plant species under different fertilization levels is not well characterized. The current study utilized a de Wit replacement series incorporating three ratios of I. batatas and M. micrantha densities and four different fertilizer levels in 16 m2 plots. In mixed culture, the total shoot length, branch, leaf, and biomass of M. micrantha were significantly suppressed (P < 0.05) by I. batatas. With increasing fertilization, total shoot length, branch, leaf, and biomass of I. batatas and M. micrantha were significantly increased, but at a greater rate for I. batatas than for M. micrantha. Relative yield parameters demonstrated that intraspecific competition was less than interspecific competition and that I. batatas had a higher competitive ability than M. micrantha under different fertilization levels. Fertilization significantly impacted the photosynthetic rate (Pn) of these species in June and July, whereas density ratio had greater effect on Pn of both species in August and September. Growth rates of I. batatas were markedly higher than those of M. micrantha in July and August, but less than those of M. micrantha in June and September. The concentrations of soil organic matter, and available N, P, and K in M. micrantha infested soil were significantly greater (P < 0.05) than soils where I. batatas was grown in monoculture, and fertilization levels were reduced by the presence of I. batatas in mixed culture. With increased fertilization, soil nutrient absorption increased, at a greater rate for I. batatas than for M. micrantha. Our results demonstrated that I. batatas could gain greater competitive advantage from enriched fertilization levels than M. micrantha. If I. batatas is incorporated into cropping system rotations, optimal fertilizer levels could be designed using information from our study to produce high I. batatas yields in addition to the benefits of suppressing invasive plants like M. micrantha.