Aim To investigate the effect of extracellular superoxide dismutase (EC-SOD) on oxidative stress induced by homocysteine (Hcy) in macrophages and its mechanism. Methods THP-1 monocyte was stimulated by phorbol ester for 48 hours and evolved into macrophages. The macrophages were dealt with 0,0, 0,0, 500 μmol/L Hcy for 72 hours, and adding a folate acid＋vitamin B12 (VitB12) intervention group (100 μmol/L Hcy＋30 μmol/L folate acid＋30 μmol/L VitB12). The changes of oxidative stress indexes (H₂O₂, O₂－, OH－) were detected by microplate test.The mRNA expression of EC-SOD was detected by real-time fluorescence quantitative PCR and the protein expression of EC-SOD was detected by Western blot in macrophages. EC-SOD assay kit was used for detecting EC-SOD activity. EC-SOD recombinant plasmid and interfering plasmid were constructed and transfected into cells, and expressions of EC-SOD mRNA, protein and superoxide anion were detected in macrophages. Results Compared with the control group, H₂O₂ and OH－ activities were significantly increased, and EC-SOD mRNA and protein expressions were significantly decreased in 0,0, 500 μmol/L Hcy group (P＜0.01). Compared with the control group, the EC-SOD activity was respectively decreased by 13.92%, 8.62%, 10.32% in 0,0, 500 μmol/L Hcy group (P＜0.05 or P＜0.01). Compared with the 100 μmol/L Hcy group, the expression of EC-SOD mRNA was increased by 47% in folate acid＋VitB12 intervention group.After transfection of EC-SOD recombinant plasmid and interfering plasmid, compared with the 100 μmol/L Hcy group, O₂－ content was decreased by 63.89% in EC-SOD recombinant group, while O₂－ content was increased by 33.59% in interfering fragment -596 group (P＜0.05 or P＜0.01). Conclusions EC-SOD is involved in the oxidative stress induced by Hcy in monocyte-derived macrophages. EC-SOD may play an important role in the inhibition of atherosclerosis induced by Hcy.