Abstract: Through seed germination and seedling growth experiment, aqueous extraction of Melilotus officinalis was used to test the effects of allelopathy on seedling growth and development of its own and four other weeds. The results showed that there was clear allelopathic effect of Melilotus officinalis on itself and other weeds, but its efficacy depended on species and the concentration of extraction. According to the seed germination experiment, it showed that the aqueous extraction of Melilotus officinalis reduced the germination rate of itself and green bristle grass（Setaria viridis）, while it was promoted for purslane（Portulaca oleracea） and Indian goose grass（Eleusine indica）. What’s more, the inhibition or promotion effect was significant at high concentration（p＜0.05）. The germination rate of common amaranth（Amaranthus retroflexus） significantly increased at higher concentration, but presenting an inhibition at a lower concentration. Germination potential decreased significantly with the refined extraction for Melilotus officinalis itself, green bristle grass, common amaranth, and purslane except for Indian goose grass at a high concentration（3.3%-10.0%）. In terms of plant height, it obviously promoted the growth of purslane and common amaranth as dicotyledon plants（p＜0.05） with the increased concentration; while for monocotyledonous plants like green bristle grass and Indian goose grass, even for Melilotus officinalis itself, plant height was inhibited at low concentration but promoted at high concentration. For biomass, the aboveground biomass was accumulated with the increased concentration of extraction for green bristle grass, common amaranth, and Indian goose grass（p＜0.05）, but presenting an inhibition effect to purslane at low concentration and promoting effects at high concentration which was opposite to Melilotus officinalis. The determination of the underground biomass of the weeds showed that it was promoted by the extraction for the monocotyledons, while the effect of inhibition for lower concentration and promotion for higher concentration was presented for the two dicotyledonous weeds. Anyhow, for Melilotus officinalis, the higher the concentration was, the more obvious the inhibition effect was.