Background: The causes of atrioventricular block (AVB) are different and diverse young patients, as compared to the old. However, little is known about the etiology distribution and clinical characteristics of AVB in the young group. Methods: We retrospectively analyzed clinical information for AVB patients under 50 years of age. We summarized clinical phenotypes for patients with undetermined AVB etiology, according to AVB type and cardiac-structural change, whereas those who received pacing therapy were followed up for suspected heart failure events (HFEs). Results: AVB etiology was identified in only 289 (61.4%) patients, while 38.6% still have undertermined etiology for AVB. Non-ischemic cardiomyopathy (16.6%) and complication of cardiac surgery (13.4%) were the top two etiologies. In addition, four distinct phenotypes were identified in AVB patients with undetermined etiology, of which the severe phenotype (both borderline/elevated left ventricular diameter or abnormal left ventricular ejection fraction and advanced AVB) accounted for 17%. Notably, 80.7% of patients with severe phenotype received pacing therapy. Based on a median follow-up time of 17.5 months, we found the occurrence of 16 suspected HFEs in 110 pacemaker receivers (12 were lost to follow up). Notably, the severe phenotype was associated with a higher risk of heart failure (HF) symptoms. Conclusions: AVB etiology in young patients under 50 years of age is complex and underdiagnosed. In patients with undetermined etiology, severe phenotype featuring advanced AVB and abnormal Left ventricle (LV) structure/function is associated with a higher rate of HF symptoms even after pacing therapy.