In this study, we used four crop seeds of Sorghum bicolor, Oryza sativa , Brassica campestris and Raphanus sativus as material to study the allelopathic effects of extracts from Phytolacca americana. The extracts were prepared from two types of Phytolacca americana, over ground and underground, at four concentrations, 0.5, 1.0, 2.0, 4.0 mg/mL respectively. The results showed that low concentration (0.5, 1.0 mg/mL) from the overground and underground parts could promote the germination of seeds of four crops, while high concentration (2.0, 4.0 mg /mL) inhibited seed germination. The extracts had obvious inhibitory effect on the root length and seedling length, and the inhibitory effect is enhanced with the increase of concentration; the extracts could reduce the activities of superoxide dismutase (SOD) and peroxidase (POD) in the seedlings of the all four crops, and increase the content of malondialdehyde (MDA) and the activity of catalase (CAT). It can be concluded that the water extracts from the upper and underground parts of the Phytolacca americana have obvious allelopathic effects on the studied four crops.