A 75-day pot experiment was carried out to study the effect of growth of ryegrass (Lolium multiflorum L.) on degradation rate of spiked phenanthrene (the concentration was 5, 50, 200 mg/kg) in soil. The results showed that ryegrass growth enhanced the degradation of phenanthrene spiked in the soil, thus making the content of extractable phenanthrene lower (P < 0.05) in the ryegrass planted pots than that of pots without ryegrass. In the treatments of 5, 50 and 200 mg/kg of phenanthrene, phenanthrene degradation rate reached 81.1, 90.4 and 85.0%, respectively, while in pots without ryegrass they were only 73.5, 86.2 and 67.6%, respectively, and ryegrass growth shorted phenanthrene half-life. Ryegrass growth enhanced activities of polyphenol oxidase, dehydrogenase and increased the content of microbiological biomass carbon, thus raised the degradation rate of phenanthrene in the soil. High concentration of phenanthrene inhibited soil biological activity, and in turn the effect of soil biology on phenanthrene degradation. Therefore, the findings disclose the biological and enzymological mechanisms of the plant enhancing phenanthrene degradation. It was also found that ryegrass is rather tolerant to polycyclic aromatic hydrocarbons, but high phenanthrene concentration affected ryegrass growth.