Abstract: Objective: To investigate the effect and mechanism of up-regulation of miR-21-5p on doxorubicin (DOX)-induced cardiomyocyte proliferation and apoptosis. Methods: Cardiomyocytes H9C2 were divided into control group (conventional- cultured control cells), DOX group (cultured with 1 μM DOX), NC agomir group (DOX+NC ago group), miR-21-5p agomir group (DOX+miR-21-5p ago group), NC antagomir treatment group (DOX+NC anta group), and miR-21-5p antagomir treatment group (DOX+miR-21-5p anta group). The expression of miR-21-5p was detected by reverse transcription-quantitative polymerase chain reaction (RT-qPCR), cell proliferation was detected by MTT, cell apoptosis was detected by flow cytometry, and the protein expression of C-Caspase-3, β-catenin and c-Myc was detected by western blotting. Results: Compared with the control group, the level of miR-21-5p and cell proliferation activity were decreased, apoptosis and C-Caspase-3 protein expression levels were increased, and β-catenin and c-Myc protein expression levels were decreased in the DOX group (P＜0.05). Compared with the DOX+NC ago group, the level of miR-21-5p in the DOX+miR-21-5p ago group was increased, cell proliferation activity was increased, apoptosis and C-Caspase-3 protein expression levels were decreased, and β-catenin and c-Myc protein expression levels were increased (P＜0.05). Conclusion: Up-regulation of miR-21-5p promotes DOX-induced cardiomyocyte proliferation and inhibits DOX-induced cardiomyocyte apoptosis through Wnt/β-catenin signaling pathway, thereby ameliorating DOX-induced cardiotoxicity.