In view of the impacts of heavy metal pollution of soil on crop growth and food safety, by adopting water culture method, the changes in physiological indexes such as mitosis, peroxidase (POD) and catalase (CAT) activities of onion roots and root tip cells under different kinds and concentrations of heavy metals were studied, with the mechanism of heavy metals on onion roots summarized. The results showed that Pb and Cd inhibited the growth of onion roots, which increased with the increase of the concentration. Under Pb and Cd stress, most of onion root tip cells were in the interphase and prophase of mitosis. Higher concentration of Pb and Cd made the nuclei of onion root tip cells distorted; The activities of POD and CAT increased with the increase of Cd concentration, but decreased with the increase of Pd concentration. Onion roots could produce corresponding physiological response mechanism to the stress of heavy metals of lead and cadmium, which provided theoretical basis and objective basis for the study of heavy metal pollution on plants.