Acidic copper mine tailings with high levels of heavy metals and arsenic pose a great risk to ecosystems and human beings. Physiological responses and phytoaccumulation of heavy metals and arsenic in Medicago sativa L. growing in soils with various proportions of acidic copper mine tailings were assessed in this study. Seed germination of M. sativa was enhanced by 5–10% tailings but inhibited at higher proportions (30% and 50%) of tailings. Seedling growth, cell membrane and photosynthesis were adversely affected when the plants were grown in soils with high proportions of tailings. The activities of antioxidant enzymes (catalase, superoxide dismutase and guaiocol peroxidase) increased with increasing proportions of tailings, indicating that the antioxidant system was not impaired and can tolerate the toxicity of mine tailings. The plants grew well in soil containing up to 50% of acidic copper mine tailings. Heavy metals and arsenic except Hg were overwhelmingly immobilized in roots. Medicago sativa is a promising plant species for revegetation and phytostabilization of acidic copper mine tailings in arid lands. However, the high mobility of Hg from root to shoot may pose risk to animals.