Abstract: In order to investigate the allelopathic effect of alfalfa (Medicago sativa L) and the relationship of water competition with the allelopathic effect under the drought condition, Root-exudates of alfalfa seedlings were extracted by dichloromethane, and identified by gas chromatography-mass spectrometry combined instrument (GC-MS). The bio-characteristics of typical exudation which were 3,5-di-tert-Butyl-4-hydroxybenzaldehyde were tested through germination experiment. The results reflected that root exudates of alfalfa mainly included straight-chain alkanes, cycloalkanes, alkenes, alcohols, methyl thiophenol, benzene, phenols, esters, heterocyclic aniline, naphthylamine, phenolic acids and polycyclic aromatic compounds. At high concentrations, 3,5-di-tert-Butyl-4-hydroxybenzaldehyde inhibited sprouting of the seedlings planted in sand more serious than the dry root weight. When water was inadequate, whether mono or mixed sowing, wheat growth was inhibited, the competition was greater than allelopathy. Under the condition of the sufficient water supplied, multiple systems were better than mono system, and allelopathic effect of intercropping showed less negative effect on harvest index than mono cropping. Intercropping of alfalfa and wheat under the stress of the dry environment, competition was greater than allelopathy.