Objectives: To explore how the mechanism of miR-18a-5p overexpression through the Notch signaling pathway improves cardiac function and inhibits cardiomyocyte apoptosis in chronic heart failure rats. Methods: Forty rats were randomly divided into contrast group, negative control (NC) group, high-expression group, and low-expression group by random number table, with ten rats in each group. NC group rats, high-expression group rats, and low-expression group rats were injected intraperitoneally with 2 mg/mL adriamycin solution to establish the rat model of chronic heart failure. The miR-18a-5p overexpression and inhibition lentivirus vector was constructed, and the miR-18a-5p overexpression and inhibition lentivirus suspension were injected into the myocardium of rats in the high-expression group and low-expression group respectively. No treatment was made in the contrast group and NC group. The expression levels of miR-18a-5p were detected in the myocardial tissue of rats and the cardiac function of rats. In addition, the apoptosis of rat cardiomyocytes was observed, and the expression levels of apoptosis-related proteins (Cleaved-caspase-3, Bax, and Bcl-2) were detected in the myocardial tissue of rats. The possible binding sites of miR-18a-5p and Notch2 3′untranslated region (UTR) region were predicted, and the interaction between miR-18a-5p and Notch2 was verified. Finally, the expression levels of Notch signal pathway-related genes and proteins (Notch2, Hes1, and Hes5) were detected in the myocardial tissue of rats. Results: In comparison with the NC group, miR-18a-5p expression levels in myocardial tissue of high-expression group rats increased (p < 0.05), and that of low-expression group rats decreased (p < 0.05). In contrast with the NC group and low-expression group, the values of left ventricular end-diastolic dimension (LVEDD) and left ventricular end-systolic dimension (LVESD) in the high-expression group decreased (p < 0.05), and the values of left ventricular ejection fraction (LVEF) and fraction shortening (FS) of the left ventricle in the high-expression group increased (p < 0.05). In comparison with the NC group and low-expression group, myocardial cell apoptosis in the high-expression group decreased (p < 0.05), and the protein expression levels of Cleaved-caspase-3 and Bax in myocardial tissue of high-expression group rats decreased (p < 0.05). Moreover, the Bcl-2 protein expression levels in the myocardial tissue of high-expression group rats increased (p < 0.05). Through prediction and verification, miR-18a-5p might bind to the “GCACCUUA” site of the Notch2 3′UTR region, which could inhibit the activity of wild-type Notch2 3′UTR luciferase reporter plasmid but could not inhibit the activity of mutant Notch2 3′UTR luciferase reporter plasmid. In comparison with the NC group and low-expression group, the messenger ribonucleic acid (mRNA) and protein expression levels of Notch signal pathway-related factors Notch2, Hes1, and Hes5 in myocardial tissue of high-expression group rats were decreased (p < 0.05). Conclusions: miR-18a-5p overexpression could improve cardiac function and inhibit cardiomyocyte apoptosis in chronic heart failure rats, which might be related to the regulation of the Notch signal pathway.