The addition of phosphorus fertilizer plays a critical role in improving alfalfa yield and quality. However, improper application may lead to resource waste and environmental pollution, and its effects are influenced by multiple factors. This study quantitatively analyzed the effects of phosphorus fertilization on alfalfa yield, crude protein (CP) content, acid detergent fiber (ADF) content, and neutral detergent fiber (NDF) content and their major influencing factors using a meta-analysis method by integrating data from published field experiments. The results showed that, compared to no phosphorus application, phosphorus fertilization increased alfalfa yield, CP content, and NDF content by 19.0% (confidence interval [CI]: 15.4–22.6%), 7.2% (CI: 0.1–14.9%), and 7.2% (CI: 0.1–14.9%), respectively, while reducing ADF content by 3.3% on average (CI: 0.9–3.3%). In Shandong, Jilin, and Hebei, where the soil pH is 7–8, annual precipitation is 200–400 mm, annual mean temperature is above 4 °C, and altitude is 500–1000 m, alfalfa yield improved after applying calcium phosphate or single superphosphate with a P₂O₅ content of 0–20% at a rate of 100–200 kg·ha⁻¹, along with nitrogen > 100 kg·ha⁻¹, and when both the test and establishment periods were 2 years. In Xinjiang, in regions with an annual mean temperature of 6–8 °C and an establishment duration of 3 years, phosphorus fertilization improved alfalfa quality. This study provides references for enhancing alfalfa productivity and efficient utilization of phosphorus nutrition.