This study addresses the challenges of limited variety, poor soil fertility, and low fertilizer efficiency in alfalfa cultivation within the desertified irrigated areas of the Hexi Corridor. Through systematic evaluation of 52 alfalfa cultivars - including adaptability assessment and N-P gradient experiments - agronomic traits, nutritional quality, and soil properties were comprehensively analyzed. Results revealed that interannual environmental variation had a greater impact on annual hay yield (AHY) and relative feed value (RFV) than cultivar differences. Principal component analysis identified fiber quality and yield potential as the main drivers of phenotypic variation. Five top-performing cultivars, such as Gannong 6 and Nongjing 8, were recommended. Nitrogen and phosphorus interaction significantly enhanced alfalfa growth and quality; the N2P2 treatment (103.5 kg ha(-)1 N, 90 kg ha(-)1 P2O5) notably increased hay yield (35.4-53.5%), crude protein (6.6-11.1%), and RFV (13.7-27.3%), while improving soil nutrient balance. The TOPSIS model validated N2P2 as the optimal strategy. This research elucidates the 'cultivar-nutrient' synergy mechanism for high-yield, quality alfalfa production, offering theoretical and practical support for sustainable intensification in arid agro-ecosystems.