Heart failure (HF) is a complex clinical syndrome resulting from the impairment of ventricular filling or ejection of blood or both, leading to considerable morbidity and mortality. Based on left ventricular ejection fraction (LVEF), the 2016 European Society of Cardiology (ESC) guideline firstly classified patients with LVEF in the range of 40% to 49% into heart failure with mid-range ejection fraction. Since then, more and more clinical studies targeting HF with mid-range ejection fraction emerged, indicating that they may benefit from similar therapies to those with LVEF \le40%. So the latest ESC guideline of HF changed the term ‘heart failure with mid-range ejection fraction’ to ‘heart failure with mildly reduced ejection fraction’ (HFmrEF). Simultaneously, burgeoning evidence indicating the emergence of novel technologies (such as speckle tracking echocardiography, cardiac magnetic resonance quantitative imaging), and new biomarkers were conducive to evaluating HF from different perspectives. In this review, we summarized the research progress of HFmrEF in clinical characteristics, prognosis, and treatment, hoping to help cardiologists better evaluate and treat patients of HFmrEF.