Three specific amino acid variations have been identified in the quasispecies of the isolated YG1 strain of severe fever with thrombocytopenia syndrome virus (SFTSV): Gn (Y328H), Gc (R624W), and L (N1891K). The Gn (Y328H) accounted for 26.9% of the viruses in the patient's blood. The other two mutations are less frequent, indicating that these mutations appeared during propagation in Vero E6 cells. To investigate the effects of each mutation on viral properties, we evaluated viruses with one to three mutations. Mutations Y328H and R624W in glycoprotein (GP) resulted in increased plaque size and cell fusion activity. Viruses with the N1891K mutation in L showed a notable cytopathic effect (CPE), which was inhibited by a pan-caspase inhibitor, suggesting that caspase-dependent cell death occurred. Programmed cell death-associated caspases were induced in both CPE-inducing and wild-type virus-infected cells. Furthermore, infection with the wild-type virus suppressed actinomycin D-induced cell death. These results suggest that SFTSV-infected cells initiate programmed cell death, whereas the wild-type virus inhibits cell death. Additionally, the recombinant single mutant virus outcompeted by a 10-fold lower amount of the wild-type virus in Vero E6 cells, indicating that the mutations were not advantageous for viral propagation in Vero E6 cells. These findings suggest that the quasispecies composition of SFTSV is influenced by the propagative environment.IMPORTANCEThis study presents findings on viral pathogenesis by analyzing quasispecies derived from a fatal case of severe fever with thrombocytopenia syndrome virus (SFTSV) infection. Analysis of recombinant SFTSV with mutations in Gn and Gc suggested that combinations of mutations may enhance the viability of mutant viruses, thereby selecting viruses to create a suitable population for propagation. The N1891K mutation in the L protein of SFTSV is associated with promoting cytopathic effects (CPE). Conversely, the wild-type virus, which is the predominant virus in infected patients, suppresses cell death. It has been suggested that SFTSV possesses a mechanism to evade cell death for prolonged viral propagation within the infected cells. Although the precise mechanism remains unknown, RNA virus polymerase may be involved in regulating cell death. This study contributes to our understanding of the mechanisms underlying the adaptation and survival of viruses as quasispecies.