Abstract Background The identification of independent risk factors for left ventricular ejection fraction (LVEF) deterioration is of clinical importance in patients diagnosed heart failure with mildly reduced ejection fraction (HFmrEF). This study aimed to determine the independent risk factors of LVEF deterioration post discharge among hospitalized HFmrEF patients. Methods This retrospective study analyzed data from 807 HFmrEF patients admitted to our hospital between January 2015 and August 2020. LVEF values were assessed by echocardiography between 6 months and 1 year after discharge. Patients were followed up to a mean of 33 months. The primary endpoint was disease progression expressed as heart failure with reduced ejection fraction (HFrEF) between 6 months and 1 year post discharge. The secondary endpoint was all-cause death. Multivariate logistic regression analysis was conducted to identify independent risk factors associated with LVEF deterioration. Results There were 225 patients progressed to HFrEF between 6 months and 1 year post discharge. All-cause death was significantly higher in HFrEF patients (43%) compared to patients with stable LVEF (unchanged or improved, 24%, P<0.001). Multivariate logistic regression analysis showed that the independent risk factors associated with worsening LVEF in patients with HFmrEF were: female (OR=1.71, 95% CI 1.14 to 2.57, P=0.009), higher NT-proBNP (≥2312 pg/ml, OR=1.71, 95% CI 1.14 to 2.57, P=0.010), higher uric acid (≥318 µmol/L, OR=11.63, 95% CI 7.89 to 17.15, P<0.001), and larger end-diastolic left ventricular dimension (OR=1.05, 95% CI 1.02 to 1.08, P<0.001). Conclusion Results of this study might help risk stratification to identify hospitalized HFmrEF patients with increased risk of progression to HFrEF post discharge. Patients with above clinical features should undergo more intensively monitoring and enhance care adherence to HF guideline medications. Future studies are needed to validate if tailored guideline adherence management strategies for post-discharged HFmrEF patients based on personalized risk stratification could mitigate LVEF progression and improve survival of these patients.