Abstract: Rice (Oryza sativa L.) seedlings were hydroponically cultivated in different concentrations of carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) (0, 2.5, 5.0, and 10.0 mg/L), 50 mmol/L mixed salt (1NaCl:9Na₂SO₄:9NaHCO₃:1Na₂CO₃), and mixed salt + MWCNTs-COOH for 10 d, respectively. Several physiological and biochemical parameters were then determined to investigate the phytotoxicity and ecotoxicological risks of MWCNTs-COOH and joint exposure with mixed salt on the seedling leaves. Results showed that reactive oxygen species (ROS), including superoxide radical (O₂^·-) and hydrogen peroxide (H₂O₂), were induced non-significantly by single MWCNTs-COOH treatment compared with the control. However, O₂^·- and H₂O₂ were overproduced in the mixed salt treatment group and in the combined treatment group. Mixed salt combined with MWCNTs-COOH aggravated the accumulation of O₂^·- and H₂O₂, with obvious concentration effects. As signaling molecules, ROS enhanced, at least to some extent, the total activities of antioxidant enzymes (i.e., SOD, CAT, POD, and APX) in the different treatment groups. The contents of chlorophyll-a and carotene somewhat increased under low concentration combined treatment compared with mixed salt treatment. After combination with mixed salt, the synthesis of soluble sugar and proline were inhibited, whereas the relative electrical conductivity and production of malondialdehyde (MDA) were significantly enhanced. The increased activity of antioxidant enzymes and production of chlorophyll-a and carotene are likely crucial defense mechanisms, which are beneficial for the alleviation of oxidative stress and damage, as well as for the maintenance of photosynthetic electron transport and thermal dissipation of excessive light energy in O. sativa seedlings. This study demonstrated that single MWCNTs-COOH treatment caused a certain level of oxidative stress and defense response in the leaves of O. sativa seedlings; furthermore, combined MWCNTs-COOH and mixed salt treatment aggravated oxidative stress and damage.