To investigate mutagenic effects of polycyclic aromatic hydrocarbons (PAHs) degradation plant under phenanthrene stress and to test the germination level of alfalfa after ion-implantation alfalfa seeds were irradiated with N,+, beam. The germination percentage and the root length distribution of alfalfa, which grew in environment with different mass fraction of phenanthrene was investigated, respectively. The results indicated that the relation of dose and germination rate was shown as "saddle" curve characteristic. It was found that the 5×10,15, cm,-2, would be the best dose of implantation. And the longest root length was about 10.32 cm. The experiment also showed phenanthrene would have inhibitive effect on germination percentage and root growth of alfalfa. The stress resistance of PAHs with alfalfa could be enhanced by ion implantation.