Abstract: The contents of phenolic acids in the extracts of soil and roots of continuously cropped for five years and normally cropped soybean in Heilongjiang province and their biological effects were studied with HPLC method.The result showed that the contents of p-hydroxy benzoic acid,vanillic acid(extracted by 1mol稬^-1 NaOH) in continuous cropping soil were significant higher than that in normal cropping soil,and the content of vanillin had no significant difference in these soils.The contents of p-hydroxybenzoic acid,vanillic acid,vanillin,ferulic acids,and coumarin of aqueous extracts of soyhean roots were higher in continuous cropping than in normal cropping soil.The activity of polyphenol oxidase in continuous cropping soil was higher than in normal cropping soil.Under conditions of water culture and adding exogenous p-hydroxybenzoic acid,the aqueous extract of soybean roots inhibited the growth of soybean seedlings.After a week of exogenous phenolic acids addition,the residual rates of p-hydroxybenzoic acid,vanillic acid,vanillin,benzoic acid,ferulic acids,and coumarin were 10.4%, 15.3%, 4.1%, 2.3%, 5.2%, 17.5%, respectively.There was a very significant exponential relationship between the addition amount of exogenous phenolic acids and the fungi population in soil.