Abstract: Objective To investigate the changes of aldehyde dehydrogenase 2 (ALDH2) expression in H2O2 inducedcardiomyocytes oxidative stress injury. Methods Cultured H9C2 cardiomyocytes were exposed to H2O2-induced oxidative stress and the effects of the ALDH2 agonist Alda-1 and ALDH2 inhibitor Daidzin were tested on the stress level of the exposed cells. MTT colorimetric assay was used to assess the cell viability after the treatments. The oxidative stress level in the myocardial cells was detected using DHE fluorescence staining, and the activity and protein level of ALDH2 were detected with spectrophotometry and Western blotting. Results Compared with normal control cells, Alda-1 treatment did not significantly affect the cell viability, oxidative stress level, or ALDH2 activity and protein level. H2O2 exposure significantly lowered the cell activity and ALDH2 activity and protein expression and increased the oxidative stress level; Alda-1 treatment obvious antagonized the effects of H2O2. Blocking ALDH2 with Daidzin produced similar effects to H2O2 exposure on the viability, oxidative stress level, and ALDH2 activity and expression in the myocardial cells. Conclusion H2O2 exposure lowers the activity and reduces the protein expression of ALDH2 in cardiomyocyte H9C2 cells, and activation of ALDH2 can alleviate H2O2-induced oxidative stress in the cells.