Effect of application of phosphorus fertilizers at different levels on photosynthesis and herbage yield and quality of alfalfa was studied.The results indicated that plants chlorophyll and photosynthesis rate were significantly increased.And then plant height and side branches of alfalfa were significantly higher in treatments with P application,compared with that in CK treatment.Plants height of alfalfa was increased by 3.54 cm and number of side branches increased by 3.78 per plant when fertilization P2O5 was 108 mg/kg.Herbage yield of alfalfa increased 25% all the year.There was significantly increased in content of crude protein,and significantly decreased crude fiber by P application properly.However,compared to the treatment without P application,crude protein was obviously increased 4.87% and crude fiber was obviously decreased 5.85% by applicating P2O5 108 mg/kg.The optimum P application rates based on benefit and yield was 231 kg/hm2(106 mg/kg) under this experimental condition.