The Angiotensin-Converting Enzyme-2 (ACE2) gene, located on Xp22.2, attracts a great deal of attention because the protein it encodes is believed to be the functional cellular receptor for the new coronavirus (SARS-CoV-2). However, recent studies are controversial, especially concerning the intrinsic link between ACE2 diversity and COVID-19 susceptibility. Here, we conduct a population genetic study on ACE2 in 6354 individuals representing 210 present-day populations and 5329 individuals of ancient or archaic groups. We dissected the genetic architecture of ACE2 and identified two major haplogroups (hg) in East Asians, i.e. ACE2-hg1 (43%) and ACE2-hg2 (53%), while other populations harbor more diverse ACE2-hgs. Accordingly, there was a significant loss of ACE2 common variations in East Asians in contrast to the X-chromosome-wide and genome-wide patterns. Notably, association analysis between ACE2-hgs and COVID-19 severity in 1229 Han Chinese individuals with various levels of COVID-19 severity showed a higher risk of ACE2-hg1 (odds ratio = 1.56, P < 0.01) and a lower risk of ACE2-hg2 (odds ratio = 0.65, P < 0.01). Interestingly, ACE2-hg1 is in strong linkage disequilibrium with rs1849863-C, which is an assumed risk factor of elevated plasma ACE2 level and is related to a higher risk of COVID-19 severity, hospitalization and infection. Strikingly, remarkable signatures of positive selection were detected, especially on ACE2-hg2, and were traced back to 100 000 years ago (but rose to a strong level during the Bronze Age, 5000 similar to 3000 years ago, in East Asians). The selection pressures could have stemmed from multiple sources, but pre-COVID-19 viral epidemics and pandemics might have been potential driving forces, which consequently contributed to the genetic susceptibility to COVID-19 within and between populations.