Alfalfa mixture is becoming increasingly important for forage production and animal husbandry development in the arid regions on the Loess Plateau of China. However, changes in the absorption and use of nutrients such as phosphorus (P) in the aged mixtures remain unclear, which may be one of the key factors controlling the variation of mixture community and productivity. This study aimed to investigate whether and how mixing ratio (i.e., species proportion) affected crop P uptake in aged alfalfa/smooth bromegrass mixtures established in 2017. Mixing with alfalfa significantly increased P concentration by 42.1% and annual P uptake by 59.8% of smooth bromegrass compared to smooth bromegrass monoculture, while mixing with smooth bromegrass imposed little impact on alfalfa P concentration and decreased annual P uptake compared to alfalfa monoculture. Mixing of crops lowered P use efficiency compared to sole cropping. Effects of mixing ratio on P concentration, uptake and use efficiency were affected by stand age and cut of mixture. Alfalfa showed a stronger competition in acquiring P than smooth bromegrass, and P competition ratio under higher alfalfa proportions (i.e., 70%) shifted from the strongest in 2021 to the weakest in 2023. Mixing of crops increased bulk soil available P content in 2022 and decreased it in 2023, while led to few changes in bulk soil total P content. Crop P traits were dominantly correlated with bulk soil available P content rather than bulk soil total P content, which could be fitted by linear or quadratic functions and varied among mixing ratios. Alfalfa/smooth bromegrass mixture still showed advantages in regulating soil P availability and crop P absorption at stand ages of 5-7 year-old. This study provides a theoretical basis for optimizing management strategy such as mixing ratio to sustain the production and resource use in aged perennial mixtures, and the use duration in the arid rainfed regions.