This is a two-center, prospective randomized controlled trial. The aim of this study is to compare the clinical efficacy of early left bundle branch pacing for cardiac resynchronization therapy and guideline-directed medical therapy in heart failure with mild-reduced ejection fraction.

Early-resync is a two-center, prospective randomized controlled trial that is designed to determine whether early left bundle branch pacing has a better impact on LV function improvement as compared with traditional guideline-directed medical therapy in heart failure with mild-reduced ejection fraction（36%≤LVEF≤50%） and complete left bundle branch block (CLBBB). Patients with symptomatic heart failure,36%≤LVEF≤50%, NYHA function class II-IV, and CLBBB（according to STRAUSS criteria) will be 1:1 randomized to LBBP+GDMT or GDMT group after enrollment and be followed at 3 and 6 months after randomization. LBBP will be performed by a double-chamber device in LBBP+GDMT group. All patients will receive GDMT. The primary endpoint is the change in the LVEF at 6 months after randomization from baseline.

Inclusion Criteria:

1. Age ≥ 18 years;
2. symptomatic heart failure despite guideline-directed medical therapy for at least 3 months, NYHA class II-IV, with an EF between 35% and 50%;
3. complete LBBB meeting Strauss's standard definition
4. Signed informed consent.

Exclusion Criteria:

1. Expected survival less than 24 months;
2. Indicated for ICD or pacing therapy;
3. History of VT, VF, or hemodynamic instability;
4. History of mechanical tricuspid valve replacement;
5. Ischemic cardiomyopathy scheduled for CABG and PCI within 3 months;
6. Severe structural heart disease may necessitate cardiac surgery or heart transplantation within 1 year;
7. Pregnancy or planning for pregnancy;
8. Hypertrophic cardiomyopathy or those underwent ventricular septal defect repair, in whom the success of LBBP is anticipated to be challenging;
9. Severe renal dysfunction (eGFR < 15ml/min*1.73m^2).