Carbon nanotubes (CNTs) have been widely used in many scientific fields including plant sciences due to their unique physical and chemical properties. However, little is known about the toxic effects of CNTs combined with cadmium (Cd) on wheat. The aim of this study was to investigate the effects of single-walled carbon nanotubes (SW) and multi-walled carbon nanotubes (MW) on the phytotoxicity of Cd in wheat. A hydroponic culture was carried out to study wheat seedling growth in six treatments, namely Cd only (Cd); MW only (MW); SW only (SW); SW combined with Cd (SWCd); MW combined with Cd (MWCd); and a control (neither Cd nor carbon nanotubes). Compared with the Cd, SW/MW alone, CNTsCd treatments induced a reduction in total root length, root surface area, average root diameter, number of root hairs, and the dry weight of shoots and roots, which indicated that wheat growth and development was significantly inhibited. In addition, an obvious decrease in tubulins in the roots was observed. However, SW/MWCd induced a significant increase in glutathione S-transferase and cyochrome P450 in the shoots and roots, which indicated that the defense ability of wheat seedlings had improved, thus alleviating Cd stress. Moreover, Cd content increased significantly in shoot and root tissues with an increase in SW/MW content, compared to the Cd treatment. According to the transmission electron microscopy, CNTs alone destroyed the cell structure, and this devastating phenomenon was deepened after combining Cd and CNTs due to CNTs carrying Cd to attack cells. Compared with MW, SW had a greater effect on wheat seedlings. To conclude, CNTs increase the toxicity of Cd to wheat seedlings. These results are significant as they evaluate indirect phytotoxicity of CNTs for adsorbing heavy metals and plant growth regulators. In view of the widespread exposure of agricultural crops to Cd, the nanotoxicity of CNTs should be seriously considered in relation to food security in the future.