The return of maize straw back to the soil is one way to achieve sustainable development in agriculture in North China, one of the major maize producing areas. This method can not only improve the soil's physical, chemical and effective bioavailability properties but also release phenolic acids recognized as allelochemicals during the decomposition process of maize straw. Phthalic acid is one of the phenolic acid decomposition products. In order to provide the basis of allelopathic effects to quantify the amount of maize straw to the field and to aid the construction of Vegetable-Maize rotation systems, the allelopathic effects of the main maize straw decomposition product (phthalic acid) on the vegetable crop used for follow-up cultivation were studied. In order to simulate the condition of maize straw returned to the field, the effects of different concentrations(0.05、0.5、1.0、2.0 mmol/L)of phthalic acid on the seed germination, seedling growth, membrane lipid peroxidation and osmoregulation substance of radish (Raphanus sativus L.) were studied in this paper, using distilled water as a control. The results showed that: (1) The degree of the allelopathy response of radish to phthalic acid was different at different growth stages. In the concentration range of 0.05-1 mmol/L, phthalic acid promoted the seed germination of radish, but the effect gradually decreased with an increased concentration of phthalic acid. When the concentration reached 2.0 mmol/L, phthalic acid had an inhibitory effect on the radish seeds'germination. (2) Phthalic acid, with 0.05 mmol/L treatment, promoted fresh biomass, dry matter accumulation and root growth of radish seedlings. The root length and root surface area of radish were significantly increased by 42.03% and 38.36%, respectively, when compared to those of the control. Compared with the control, the activity of superoxide dismutase (SOD) increased, but activities of peroxidase (POD) and catalase (CAT) decreased in the radish seedlings. There were no significant differences in the malondialdehyde (MDA) content between the treated and control radish seedlings. (3) When the concentration of phthalic acid was higher than 0.5mmol/L, the degree of damage to the membrane lipid peroxidation of the radish seedlings increased and MDA contents sharply increased. Metabolic and physiological function disorder was found, and the plant growth and matter accumulation of radish seedlings were significantly inhibited. When the concentration of phthalic acid was 2 mmol/L, leaf and root tip numbers, root length and root surface area of the radish reduced by 36.51%, 41.28%, 64.46% and 40.20%, respectively, when compared with the control. (4) The influence on osmotic adjustment substance showed that phthalic acid with different concentrations in the four treatments all promoted the accumulation of soluble sugar content in radish seedling leaves, but the promotion effect gradually decreased with an increased concentration of phthalic acid. With an increasing concentration of phthalic acid, the soluble protein content gradually decreased by 12.82%, 14.88%, 21.58% and 24.73%, over the four treatments, respectively, when compared to the control. If only considering the allelopathic effects to ensure the best implementation of Maize-Vegetable rotation and quantization of returning maize straw to the field in North China, the phthalic acid concentration should be less than 0.5mmol/L to prevent the negative influences of high concentrations of phthalic acid on the growth of radish seedlings.