Plant invasion has severely damaged ecosystem stability and species diversity worldwide. The cooperation between arbuscular mycorrhizal fungi (AMF) and plant roots is often affected by changes in the external environment. Exogenous phosphorus (P) addition can alter the root absorption of soil resources, thus regulating the root growth and development of exotic and native plants. However, it remains unclear how exogenous P addition regulates the root growth and development of exotic and native plants mediated by AMF, affecting the exotic plant invasion. In this experiment, the invasive plantEupatorium adenophorumand native plantEupatorium lindleyanumwere selected and cultured under intraspecific (Intra-) competition and interspecific (Inter-) competition conditions, involving inoculation with (M+) and without AMF (M−) and three different levels of P addition including no addition (P0), addition with 15 mg P kg−1soil (P15), and addition with 25 mg P kg−1soil (P25) for the two species. Root traits of the two species were analyzed to study the response of the two species’ roots to AMF inoculation and P addition. The results showed that AMF significantly promoted the root biomass, length, surface area, volume, tips, branching points, and carbon (C), nitrogen (N), and P accumulation of the two species. Under M+treatment, the Inter- competition decreased the root growth and nutrient accumulation of invasiveE. adenophorumbut increased the root growth and nutrient accumulation of nativeE. lindleyanumrelative to the Intra- competition. Meanwhile, the exotic and native plants responded differently to P addition, exhibiting root growth and nutrient accumulation of invasiveE. adenophorumincreased with P addition, whereas nativeE. lindleyanumreduced with P addition. Further, the root growth and nutrition accumulation of nativeE. lindleyanumwere higher than invasiveE. adenophorumunder Inter- competition. In conclusion, exogenous P addition promoted the invasive plant but reduced the native plant in root growth and nutrient accumulation regulated by AMF, although the native plant outcompeted the invasive plant when the two species competed. The findings provide a critical perspective that the anthropogenic P fertilizer addition might potentially contribute to the successful invasion of exotic plants.