The Eurasian avian-like swine (EA) H1N1 virus has been widely prevalent in the Chinese swine population and has caused infections in human. However, knowledge regarding its pathogenic mechanisms remains limited. In this study, we analyzed the pathogenic determinants of two G4 genotype EA H1N1 viruses (A/Swine/Guangdong/SS12/2017 and A/Swine/Jiangxi/1110/2017) with differing pathogenicity by constructing a series of reassortant and mutant viruses. The HA-G219A mutation was found to be determinant of pathogenicity in mice. Subsequent analyses revealed that this mutation enhances viral replication in human cells, improves thermal stability, reduces HA activation pH, and alters receptor-binding properties. Furthermore, HA-G219A mutation may be an adaptive mutation that facilitates influenza virus adaptation to swine, with its prevalence increasing in the swine population. This mutation may support cross-species transmission of EA H1N1 swine influenza viruses or genetic exchange with other virus subtypes/genotypes, potentially contributing to the emergence of pandemic viruses. These findings improve our understanding of EA H1N1 pathogenicity and highlight the critical need for ongoing surveillance of influenza viruses in pigs.