Human metapneumovirus (hMPV) is a respiratory viral pathogen in children worldwide. hMPV is divided into four subtypes: hMPV_A1, hMPV_A2, hMPV_B1, and hMPV_B2. hMPV_A2 can be further divided into hMPV_A2a and A2b based on phylogenetic analysis. The typical prevalence pattern of hMPV involves a shift of the predominant subtype within one or two years. However, hMPV_A2, in particular hMPV_A2b, has circulated worldwide with a several years long term high epidemic. To study this distinct epidemic behavior of hMPV_A2, we analyzed 294 sequences of partial G genes of the virus from different countries. Molecular evolutionary data indicates that hMPV_A2 evolved toward heterogeneity faster than the other subtypes. Specifically, a Bayesian skyline plot analysis revealed that hMPV_A2 has undergone a generally upward fluctuation since 1997, whereas the other subtypes experienced only one upward fluctuation. Although hMPV_A2 showed a lower value of mean dN/dS than the other subtypes, it had the largest number of positive selection sites. Meanwhile, various styles of mutation were observed in the mutation hotspots of hMPV_A2b. Bayesian phylogeography analysis also revealed two fusions of diffusion routes of hMPV_A2b in India (June 2006) and Beijing, China (June 2008). Sequences of hMPV_A2b retrieved from GenBank boosted simultaneously with the two fusions respectively, indicating that fusion of genetic transmission routes from different regions improved survival of hMPV_A2. Epidemic and evolutionary dynamics of hMPV_A2b were similar to those of hMPV_A2. Overall, our findings provide important molecular insights into hMPV epidemics and viral variation, and explain the occurrence of an atypical epidemic of hMPV_A2, particularly hMPV_A2b.