A 75-day-long pot experiment was carried out to study effect of plantation of ryegrass (Lolium multiflorum) on dynamic degradation of phenanthrene (PHE) in soil. Results indicate that ryegrass plantation enhanced degradation of PHE in the soil, thus making the content of extractable PHE lower (p<0.05) in the ryegrass pots than in CK (pots without ryegrass). In the treatments, 5, 50, and 200 mg kg^-1 in PHE concentration, PHE degradation rate reached 81.07%, 90.35% and 84.94%, respectively, while in CK it was only 73.5%, 86.2% and 67.60%, respectively. Ryegrass plantation enhanced activities of polyphenol oxidase, dehydrogenase and catalase, and increased content of microbiological biomass C, that is, the soil biological activity, thus raising the degradation rate of PHE in the soil. And soil biological activity varied sharply from treatment to treatment. Higher concentration of PHE inhibited soil biological activity, and in turn the effect of soil biology on PHE degradation. Thus, the findings disclose the biological and enzymological mechanisms of the plant enhancing PHE degradation. And it was also found that ryegrass is rather tolerant to polycyclic aromatic hydrocarbons, but higher PHE concentration affected ryegrass growth.