Abstract: 【Objective】 Low soil available phosphorus content is one of the main factors restricting crop growth. This project aims to clarify the effect of phosphorus level on alfalfa root morphology and plant growth in the hope of providing a theoretical basis for the study of alfalfa tolerant mechanisms. 【Methods】 In this experiment, under hydroponic conditions, six kinds of phosphorus were analyzed, which were 0 umol/L (P₁), 50 umol/L (P₂), 100 umol/L (P₃), 250 umol/L (P₄), 500 umol/L (P₅), 1000 umol/L (P₆) levels affecting the root characteristics, plant growth and nutrient accumulation of Xinmu 1 alfalfa at the seedling stage. 【Results】 With the increase of phosphorus level, the root-to-shoot ratio decreased significantly (P<0.05), and the biomass first increased and then decreased. Except for the P₄ and P₅ treatments, there was no significant difference in plant height (P<0.05), and there was no significant difference among other treatments (P<0.05). The plant height was the highest under P₄ treatment, which was 9.37 cm; the total root length, root surface area and root-to-shoot ratio reached the maximum under P₂ phosphorus supply level, which were 281.75 cm, 20.78 cm², and 1.13; 6 species, under the phosphorus level, the accumulation of nitrogen, phosphorus, and potassium in the ground were greater than the accumulation in the ground, and comprehensive comparison showed that the P₅ treatment was more conducive to alfalfa growth. 【Conclusion】 Low phosphorus concentration can promote the growth of alfalfa roots, while excessively high phosphorus concentration inhibits its growth, reduces plant height, total root length, root surface area and root-to-shoot ratio.