In a previous outbreak of the human respiratory syncytial virus (HRSV), we identified a variant strain of genotype BA9 with a seven-amino-acid extension (Q-R-L-Q-S-Y-A) at the C-terminus of the attachment protein (G). To assess the impact of this extension on the virulence of HRSV, two full-length infectious clones using the wild strain of genotype BA9 as a backbone, one containing the seven-amino-acid extension (rRSV BA9 WT), and the other deleting this extension (rRSV BA9 Δ7AA), were successfully rescued using a reverse genetics system. The biological properties and virulence of the two rescued viruses were then compared and analyzed in vitro and in vivo. Compared to the rRSV BA9 Δ7AA, the rRSV BA9 WT exhibited a larger plaque size and a more pronounced suppression of the host cell innate immune response in vitro (IFN-β levels: 154.33 pg/mL vs. 11.27 pg/mL). The rRSV BA9 WT demonstrated increased adaptability in mice, with a 10-fold higher lung viral load and a stronger inflammatory response following intranasal exposure. Our study primarily demonstrated that the C-terminal extension of the G protein of the HRSV can enhance viral virulence, underscoring the importance of virological surveillance in the prevention and treatment of severe HRSV-related disease.