250 words 25 Importance: 150 words 26 Abstract 28 A novel coronavirus SARS-CoV-2 is associated with the current global pandemic of Coronavirus 29Disease 2019 . Spike protein receptor-binding domain (RBD) of SARS-CoV-2 is the 30 critical determinant of viral tropism and infectivity. To investigate whether the mutations in the RBD 31 have altered the receptor binding affinity and caused these strains more infectious, we performed 32 molecular dynamics simulations of the binding affinity between the mutant SARS-CoV-2 RBDs to 33 date and the human ACE2 receptor. Among 1609 genomes of global SARS-CoV-2 strains, 32 34 non-synonymous RBD mutants were identified and clustered into 9 mutant types under high positive 35 selection pressure. Three mutant types (V367F, W436R, and D364Y) emerging in Wuhan, Shenzhen, 36 Hong Kong, and France, displayed higher human ACE2 affinity, and probably higher infectivity. This 37 is due to the enhanced structural stabilization of the RBD beta-sheet scaffold. High frequencies of 38 RBD mutations were identified: V367F from five France and one Hong Kong mutants, 13 V483A and 39 7 G476S mutants from the U.S.A. This suggested they originated as novel sub-lineages. The 40 enhancement of the binding affinity of the mutant type (V367F) was further validated by the 41 receptor-ligand binding ELISA assay. The molecular dynamics simulations also indicated that it 42 would be difficult for bat SARS-like CoV to infect humans. However, the pangolin CoV is potentially 43 infectious to humans. The analysis of critical RBD mutations provides further insights into the 44 evolutionary history of SARS-CoV-2 under high selection pressure. An enhancement of the 45 SARS-CoV-2 binding affinity to human ACE2 receptor reveals higher infectivity of the mutant 46 strains. 47 48 Importance 49 A novel coronavirus SARS-CoV-2 has caused the pandemic of COVID-19. The origin of 50 SARS-CoV-2 was associated with zoonotic infections. The spike protein receptor-binding domain 51 (RBD) is identified as the critical determinant of viral tropism and infectivity. Thus, whether the 52 mutations in the RBD of the circulating SARS-CoV-2 strains have altered the receptor binding 53affinity and caused these strains more infectious, should be paid more attentions to. Here, 32 54 non-synonymous RBD mutants were identified and clustered into 9 mutant types under high positive 55 . CC-BY-NC-ND 4.0 International license (which was not certified by peer review) is the author/funder. It is made available under a selection pressure, suggesting they originated as novel sub-lineages. Three mutant types displayed 56 higher human ACE2 affinity, and probably higher infectivity, one of which (V367F) was validated 57 by wet bench. The RBD mutation analysis provides insights into SARS-CoV-2 evolution. The 58 emergence of RBD mutations with increased human ACE2 affinity reveals higher risk of severe 59 morbidity and mortality during a sustained COVID-19 pandemic, particularly if no effective 60 precautions are implemented. 61 62