This study, in taiyuan in Shanxi Province's northern suburbs surrounding soil as the research soil, studied the different treatment levels of Pb and Hg composite stress effect toxicity of alfalfa seed, chlorophyll content, relative electric conductivity and enzyme (SOD, POD, CAT) activity in vivo. The results showed that the increase of Pb-Hg compound concentration decreased seed germination rate of alfalfa, and it had a certain toxic effect on it. With the increase of Pb-Hg concentration, the height of the plant increased first and then decreased. The conductivity gradually increased and the permeability of plant cell membrane decreased gradually. The chlorophyll content of leaves decreased gradually. The content of MDA gradually increased, the SOD enzyme increased and then decreased, but the POD enzyme decreased, and the activity of CAT enzyme decreased gradually. The results show that alfalfa has some tolerance for the soil of Pb-Hg.