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The velvetleaf (<i>Abutilon theophrasti</i> Medik.) is a strong and competitive weed in fields that inhibits the growth of crops. Reports have suggested that allelopathy is one of the reasons for this inhibition; however, the mechanism of this allelopathy remains unclear. In this study, velvetleaf powder/extracts were shown to inhibit seed germination, growth and yield in maize, wheat and soybean through petri dish, pot and field control experiments. We observed a concentration-dependent inhibition of the seed germination rate for all three crops. The root tip structure changed significantly and the embryo even died when irrigated with a high concentration of the extract (10 mg·mL⁻¹). After adding velvetleaf powder, the malondialdehyde (MDA) content in crop seedlings was dose-dependent, and the superoxide dismutase (SOD) activity of maize, wheat and soybean showed the maximum values under treatment with 1.25, 5 and 5 mg·cm⁻³, respectively. The activity of peroxidase (POD) showed the highest value under the 5 mg·cm⁻³ treatment in maize and wheat seedlings and under 10 mg·cm⁻³ treatment in soybean seedlings. However, sugar, protein content and root activity in all three crops was the lowest under the 10 mg·cm⁻³ treatment. Therefore, velvetleaf may decrease the productivity of three crops by changing the antioxidant enzyme activities, root system activities and root tip structures.