Polycyclic aromatic hydrocarbons (PAHs) is one kind of persistent organic pollutants in environment, some previous researches have indicated plant could facilitate the dissipation of PAHs in contaminated soil. Phytoremediation is an important measure to remove organic pollutants from contaminated soil, and the root secretion of plant is considered to be closely related to the mechanisms of phytoremediation to organic pollutants. It is in favor of revealing the mechanisms of remediation by studying the characteristics of root exudates of plant with phytoremediation potential under the stress of pollutant. In the present research, pyrene and ryegrass (Lolium perenne L.) which has been testified to be tolerant to PAHs dtress were selected as studied objects. A solution culture experiment with series of pyrene concentration was conducted to investigate the effects of pyrene on low molecule weight organic compounds in the root exudates of ryegrass (Lolium perenne L.). The series concentration of pyrene in culture solution were designed as 0 mg/L, 3 mg/L, 6 mg/L and 9 mg/L. The results showed that, ryegrass had stronger tolerance to pyrene stress and grew well in culture solution spiked with 9 mg/L pyrene. Despite the biomass raised appreciably at relative low pyrene concentrations treatment (<3 mg/L) and reduced at relative high pyrene concentrations treatments (>3 mg/L), no significant changes of ryegrass biomass were found in all pyrene stress treatment (P<0.05). At the pyrene spiked levels of 3 mg/L, 6 mg/L and 9 mg/L pyrene stress, the organic acids, including oxlic acid, lactic acid and malic acid, were found in the root exudates of ryegrass, such component of organic acid in root exudates of ryegrass was stabilization in all pyrene stress treatment, but oxlic acid was the main composition of organic acid in root exudates of ryegrass, with a percentage greater than 97%, and its content increased significantly with the increase of pyrene spiked level (P<0.05). Compared with control treatment (0 mg/L), oxlic acid content in root exudates of ryegrass increased from 24.6 mg/kg to 53.7 mg/kg in 9 mg/L pyrene treatment. The content of carbohydrate in root exudates of ryegrass raised appreciably at relative low pyrene spiked level (<3 mg/L) and reduced at relative high pyrene spiked level (>3 mg/L), the highest carbohydrate content in root exudates took on 3 mg/L pyrene treatment, but the differences of carbohydrate content in root exudates were insignificant in all pyrene treatments (P<0.05). Total amount of amino acids secreted increased significantly with the pyrene concentration increase, which were 1.37, 2.02 and 2.65 times than that of control treatment (0 mg/L) under 3 mg/L, 6 mg/L and 9 mg/L pyrene treatment,respectively. Data also indicated that 19 types of familiar amino acids were found in root exudates of ryegrass and the component of amino acids in root exudates of ryegrass was stabilization under all pyrene stress treatments, but the each content of 19 types of amino acids examined was different in those root exudates under the stress of different pyrene concentration. The contents of all amino acid in those root exudates was increased with the pyrene concentration increase, especially, the contents secreted of threonine, serine, proline, glycine, alanine, histidine and ornithine increased significantly among 19 types of amino acids and the differences were significantly among different treatments with different pyrene concentration (P<0.05).