Abstract: To study the effect of mowing time on the reproductive growth and provide basic data for improving seed yield of alfalfa, Medicago sativa L.‘Xinmu No.4’was used as the experiment material. Alfalfa were mowed on the 30th, 37th, 44th, 51st, 58th and 65th day, namely on April 24, May 1, May 8, May 15, May 22 and May 29 in 2013, after resuming growth. Alfalfa in the control group was not mowed. The results showed that mowing time had important effect on reproductive pattern, seed yield and seed characteristics of alfalfa. Compared with the control group, 44 (May 8) and 51 (May 15) days after resuming growth, the thousand seed weight increased by 24.44% and 30%, seed yield increased by 26.85% and 45.86%, respectively. Whereas, seed germination rate reduced by 2.12% and 10.63%, respectively. Mowing could delay flowering period of alfalfa. Mowing once during flower bud formation period to early blooming period can improve the fertilization efficiency of flowers, decrease abortive reproductive rate and increase seed yield.