Allelochemical substitutions salicylic acid, p-hydroxybenzonic acid, cinnamic acid, vanillic acid and ferulic acid were tested in a gyrating regression experiment to study the weed-suppressive effect of phenolic acids on the root growth of barnyardgrass. The results showed that cinnamic acid had a significant inhibitory effect, its square item regression coefficient being -6.18; salicylic acid, p-hydroxybenzonic acid and ferulic acid had the same trend of weed-suppressive potential as cinnamic acid, while vanillic acid had a different weed-suppressive effect from the others. The inhibition rate (IR) on the root growth of barnyardgrass reached its highest level (78.5%) when 0.06 mmol x L(-1) salicylic acid, 0.60 mmol x L(-1) p-hydrobenzonic acid, 0.24 mmol x L(-1) cinnamic acid, 0.02 mmol x L(-1) vannillic acid and 0.02 mmol x L(-1) ferullic acid were mixed together to apply.