Abstract: To understand the effects of CdCl_2stress to seed germination in alfalfa (Medicago sativa) is important for its bioremediation of heavy metal. Using the petri dish germination method, the effects of different concentrations of CdCl₂(0,5, 10,0, 40and 80mg/L) treated with alfalfa seed germination were studied under three conditions of pH (4.0,6.0and 8.0). The results showed that the treatments with low concentrations of CdCl₂(≤20mg/L) significantly improved the seed germination rate, germination potential, germination index and vigor index. Compared to the control group, the vigor index greatly increased up to 177.52±16.61under the treatments of concentration of CdCl_2was 20mg/L (pH 6.0). The promotion for root length, shoot length, root fresh weight and shoot fresh weight were also strengthened with the increasing concentration under 20mg/L CdCl₂.Treated with concentration of CdCl_2was 20mg/L under pH 4.0, pH 6.0and pH 8.0, the shoot fresh weight was (6.60±0.11), (6.68±0.20) and (9.51±0.16) mg, while root fresh weight was (6.71±0.10), (7.09±0.08) and (9.10±0.08) mg, up to the maximum respectively. The seed germination and seedling growth of alfalfa were significantly inhibited as CdCl_{2concentration} increased (40and 80mg/L). Under 80mg/L, the germination rate and vigor index of alfalfa seed were only 14.8% and 46.5% of those of controls, and shoot fresh weight decreased to (3.48±0.15), (4.03±0.19) and (7.00±0.22) mg under pH 4.0, pH 6.0and pH 8.0, all down to the minimum respectively. The germination rate and vigor index of alfalfa seed decreased with the decrease of pH. Treated with concentration of CdCl_2was 80mg/L under weak base (pH 8.0) condition, the germination rate (66.40%±3.19%) was higher than those treated with strong acid (pH 4.0) (11.20%±3.61%) and weak acid (pH 6.0) (9.20%±4.22%), while the vigor index (93.90±10.71) was also higher than those under pH 4.0(49.77±3.25) and pH 6.0(56.67±3.48), respectively. The results indicated that adjust of the environment from acid to alkaline could alleviate CdCl_2toxicity to seed germination and seedling growth in alfalfa.