Abstract: In order to assess the dynamic changes of nutrient content in alfalfa hay bales during storage, the dynamic changes of nutrient content and relative feeding value (RFV) of alfalfa hay bales with water content of 15% and bundles density of 150 kg/m3 at different storage durations (1, 10, 30, 90, 180, 360 d) were studied. The results showed that the crude protein (CP) content of alfalfa hay was decreased in the whole storage process;compared to the first day of storage, the CP content of alfalfa hay was decreased significantly in the other days of storage (P<0.05);from the perspective of CP content, the quality of alfalfa hay was decreased from grade 1 to grade 2. The content of neutral detergent fiber (NDF) of alfalfa hay was elevated with the prolongation of storage time;the content of NDF in the 360th day of storage was significantly higher than that in the 90th day of storage (P<0.05);from the perspective of NDF content, the quality of alfalfa hay was decreased from grade 2 to grade 3. The content of acid detergent fiber (ADF) was also increased with the prolongation of storage time, after 10 days of storage, the content of ADF began to increase significantly (P<0.05);from the perspective of ADF content, the quality of alfalfa hay was decreased from grade 1 to grade 4. The DMI value of alfalfa hay stored for more than 30 days was significantly lower than that within 30 days (P<0.05);from the perspective of DMI value, the quality of alfalfa hay was decreased from grade 1 to grade 3. The DDM value of alfalfa hay stored for more than 90 days was significantly lower than that stored for less than 90 days (P<0.05);from the perspective of DDM value, the quality of alfalfa hay was decreased from grade 1 to grade 4. With the prolongation of storage time, the RFV value of alfalfa hay was reduced significantly (P<0.05);the RFV value of alfalfa hay was decreased greatly during 180 -360 days of storage;from the perspective of RFV value, the quality of alfalfa hay was decreased from grade 2 to grade 3. In general, the nutritional value of alfalfa hay was significantly reduced after 180 days of storage. Therefore, the alfalfa hay was recommended to be sold or fed to livestock within 180 days of storage.