Abstract: In order to investigate the effects of phosphorus on the branching growth and the related physiology of Medicago sativa under the short-term cultivation, the non-fall dormancy variety WL525 HQ was used as the experimental material. The branching characteristics, photosynthetic indexes and root nutrients of Medicago sativa under different phosphorus levels were determined by field experiment and pot experiment. The results showed that phosphorus treatment had no significant effect on the number of root crown shoots and shoot branching of Medicago sativa, but significantly increased the single branch weight of root crown shoots and shoot branching (P<0.05), thus increasing the dry weight per plant. The phosphorus treatment improved the photosynthetic performance of Medicago sativa. Compared with the control group, the net photosynthetic rate of the first crop treated with 10 g, 15 g and 20 g P₂O₅ under the potted cultivation was significantly increased by 14.0%, 16.7% and 14.2%, respectively. The phosphorus treatment tended to increase the contents of soluble sugar, starch and total non-structural carbohydrate (TNC) in the roots of Medicago sativa, and significantly increased their storage capacity (P<0.05). The total nitrogen content and its storage capacity increased significantly with the increase of phosphorus application (P<0.05). The daily increment of dry matter of Medicago sativa after mowing was significantly increased with the increase of phosphorus application rate (P<0.05), and the regeneration rate was significantly positively correlated with the storage capacity of root nutrients (P<0.05). The comprehensive test results showed that: the phosphorus treatment has improved the dry matter yield of Medicago sativa mainly by increasing the single branch weight of branches, and the reason why phosphorus could increase the single branch weight of branches was that the phosphorus has promoted the flow of nutrients between roots and newborn branches, thus accelerating the growth of newborn branches.