Abstract: In order to investigate how phosphorus application affects the yield, nutrient value and nitrogen (N) and phosphorus (P) utilization of alfalfa, the effects of different phosphorus levels (P₀: 0 kg·ha^-1; P₁: 126 kg·ha^-1; P₂: 252 kg·ha^-1) on the performance, nutrient value, nitrogen and phosphorus utilization and key enzymes of nitrogen metabolism in alfalfa were studied, to examine the optimal nitrogen fertilizer rate under field conditions. The variety Gannong No.3 was used as experimental material. With optimal N supply, alfalfa total annual yield under P₁ was 33312 kg dry matter·ha^-1, significantly higher than in P₀ or P₂. Activities of nitrate reductase (NR) and glutamine synthetase (GS), crude protein content, total protein yield, nitrogen uptake efficiency and nitrogen production of alfalfa were all significantly higher under P₁ and P₂ than under P₀. Although often not differing significantly, values for all the above indexes were numerically higher in the P₁ treatment than under P₂. The results show that application of phosphate fertilizer to alfalfa significantly increases productivity, and improves forage quality, and N and P utilization. The application rate of P fertilizer followed the law of diminishing returns in terms of the yield, forage quality and utilization rate. In conclusion, the best P supply level was 126 kg·ha^-1 in this test. The appropriate phosphorus supply enhances plant metabolic activity by promoting the absorption and assimilation of N through the activation of NR and GS activity, thereby enhancing yield, quality and nitrogen use efficiency in alfalfa.