The allelopathic effects of water extracts from alfalfa (Medicago sativa) in the seedling stage on seed germination and growth of Elymus nutans in different areas were examined in the laboratory based on seed germination rate, seedling length, root length, and dry weight of seedlings and roots. The results showed that the seed germination rate of Elymus nutans in LS (Lhasa) and DX (Damxung County) was significantly promoted by water extracts from the aboveground alfalfa, but that of those in GS (Gansu), KMX (Kangma County), LKZ (Nagarzê County), and LZ (Nyingchi County) was inhibited, and Elymus nutans in GS was the most inhibited, with suppression rates of 57.89% and 55.26% at concentrations of 14.5% and 5.5%, respectively. At the concentration of 5.5%, the seedling length of Elymus nutans in LZ was the most inhibited with the suppression rate of 33.03%, followed successively by those in NQ (Nagqu region), DX, LS, KMX, QH (Qinghai), GS, and LKZ. At the concentration of 14.5%, the seedling length of Elymus nutans in LZ was nearly the most inhibited with the suppression rate of 28.97%, followed successively by those in KMX, LKZ, NQ, GS, QH, DX, and LS. At the concentration of 5.5%, the root length of Elymus nutans in QH had the strongest inhibition, followed by those in the order of NQ > LZ > KMX, GS > LKZ > DX, LS. At the concentration of 14.5%, the root length of Elymus nutans in GS had the strongest inhibition with the suppression rate of 57.69%, followed successively by those in the order of QH, NQ > LZ > LS > KMX > DX > LKZ. The dry weight of seedlings of Elymus nutans in LKX and LZ increased by the aboveground water extracts, whereas that of those in KMX and DX decreased and the dry weight of seedlings of Elymus nutans in NQ was only increased at high concentrations (RI > 0). The dry weight of roots of Elymus nutans in LS, QH, GS, NQ, and LKZ was decreased by the aboveground water extracts, whereas that of those in KMX and DX was increased and the dry weight of roots of Elymus nutans in LZ was only increased at high concentrations. Seed germination rate, seedling length, root length, and dry weight of seedlings and roots of Elymus nutans in most areas were significantly inhibited (RI < 0) by water extracts from the root of alfalfa, except for seed germination and dry weight of roots of Elymus nutans in LS and DX. Seed germination and dry weight of seedlings of that in GS and the dry weight of roots of that in NQ were significantly promoted (P > 0.05). All the above results showed that the sensitivity of Elymus nutans in different areas to the allelopathic effects of the aboveground alfalfa in the seedling stage is in the order of GS > QH > LS > KMX > NQ > LKZ > LZ > DX from the highest to the lowest allelopathic sensitivity. The allelopathic sensitivity to the root of the alfalfa is in the order of QH > NQ > LZ > KMX > LS,from the highest to the lowest. The allelopathic sensitivity of Elymus nutans in different areas to water extracts from the aboveground alfalfa is higher than that from the root of alfalfa.