In order to explore the allelopathic mechanism of coumarin and be closed to the actual production,this experiment used greenhouse potting method treated with different concentrations of coumarin to study the effects of coumarin on the growth and ultrastructure of Lolium multiflorum seedlings.The results showed that coumarin could significantly inhibit the growth and biomass accumulation of Lolium multiflorum at different concentrations,and higher concentration resulting in stronger inhibition.With the higher concentration of coumarin,the content of chlorophyll a,chlorophyll b and total chlorophyll were significantly decreased.Compared with the control,the relative conductivity and malondialdehyde（MDA） content of the leaves were significantly increased,Peroxidase（POD）,catalase（CAT） and superoxide dismutase（SOD） activities were significantly enhanced under the treatment of 500 mg/kg.Coumarin destroyed the ultrastructure of Lolium multiflorum leaf cells which led to chloroplast deformation,central vacuolar lysis and increasing starch granules.Coumarin affected little under the concentration of 50 mg/kg while under the concentration of 500 mg/kg the organelles were almost completely degraded.