Drought and soil nutrient deficiency are critical constraints on plant growth and ecological restoration in desert steppes; however, the interactive mechanisms between arbuscular mycorrhizal fungi (AMF) and phosphorus fertilization remain poorly elucidated. To investigate the regulatory mechanisms governing root system architecture (RSA) remodeling and regrowth capability in Agropyron under drought stress, a controlled experiment was conducted using two genotypes: Inner Mongolia (NM) and Xinjiang (XJ). The experimental design comprised three water regimes (70%, 50%, and 30% field capacity [FC]), two P levels (P0, P1), and two inoculation treatments (A0, A1). The results indicated the following: (1) Although drought significantly inhibited Agropyron growth, the combined application of AMF and P (A1P1) induced a highly significant synergistic effect, augmenting total aboveground biomass by 66.08-160.58% compared to the control. This synergy exhibited distinct "environmental dependency," being most pronounced under moderate drought conditions (50% FC). (2) Mechanistic analysis revealed that A1P1 optimized RSA by significantly increasing total root length, root surface area, and root volume (e.g., total root length increased by 281.4-375.1% under severe stress), thereby enhancing water and nutrient acquisition. (3) The A1P1 treatment significantly mitigated the decline in regrowth potential induced by successive clipping, sustaining a higher tiller number (increasing by up to 1.8-fold in the 3rd clipping). (4) The XJ genotype was characterized by higher basal biomass and root investment "high-yield phenotype", whereas the NM genotype demonstrated greater sensitivity to AMF-P regulation "highly responsive phenotype". In conclusion, the synergistic interaction between AMF and P mitigates drought stress by reshaping RSA and enhancing regrowth capability, providing a theoretical basis for the efficient management of arid grasslands.