Abstract: In the semiarid loess hilly and gully region of Northwest China, the availabilities of water and phosphorus (P) are the main limitations on seeded alfalfa (Medicago sativa L.) production. The water-harvesting technique can improve the soil water condition. However, the contribution of availability of soil phosphorus to the production of seeded alfalfa established by this technique is unknown. In this research, we studied the availability of soil phosphorus in response to the production of seeded alfalfa established by the ridges and furrows water-harvesting technique. The following five treatments were set up in this study: 1) conventional cultivation in a flat plot without mulch (CK), 2) plastic mulched ridge with 30 cm width of ridge and furrow (M30), 3) plastic mulched ridge with 60 cm width of ridge and furrow (M60), 4) bare ridge with 30 cm width of ridge and furrow (B30), and 5) bare ridge with 60 cm width of ridge and furrow (B60). The forage yield, evapotranspiration (ET), soil organic carbon (SOC), total phosphorus, available phosphorus, and the ratio of soil organic C to available P (C/P) were measured in the experiment. Results showed that the total soil P was not significant different among treatments and sampling dates. However, the positive correlation relationship occurred between the decrease in soil available P and total forage yield of alfalfa after 3-year alfalfa stand. The forage yield of alfalfa in the two ridge-mulched treatments (M30 and M60) were higher than that of conventional flat cultivation (CK) by 10.7% and 40.3%, respectively, while the yield of two bare treatments (B30 and B60) were lower than that of CK by 14.2% and 28.3%, respectively. After a 3-year alfalfa stand, the trend of the decreases in soil available P was in the order of M60 (55.5%)> M30 (51.5%)> CK (34.6%)> B30 (23.4%)> B60 (17.5%). Moreover, after 3-year alfalfa growth, the C/P ratio increased significantly in all of the treatments comparing to that before sowing. The C/P ratio of M30 (1165.1) and M60 (1326.1) were significantly higher than that of the other treatments. More interesting finding was that the increase in SOC in dry years promoted the accumulation of soil available P. However, in wet years, the increase of SOC limited the accumulation of soil available P. Therefore, to further increase the forage yield of alfalfa, the effective technique must be developed to increase the soil available P.