This study aims to investigate the gas–liquid two-phase flow characteristics during kill fluid displacement in large-diameter wellbore well control operations and their impact on wellbore pressure stability and operational safety, with a focus on the roles of wellbore size, bottomhole pressure differential, kill fluid displacement rate, and density. Employing Computational Fluid Dynamics (CFD), the Volume of Fluid (VOF) multiphase model was utilized to capture the gas–liquid interface, coupled with the SST K–ω turbulence model to simulate complex flow dynamics. Systematic simulations were conducted to analyze the transient displacement process of kill fluid under varying conditions of wellbore size, bottomhole pressure differential, kill fluid density, and inlet velocity. The simulation results indicate that no significant gas accumulation forms at the wellbore bottom during kill fluid displacement, and the invaded gas is effectively expelled upon completion of the displacement. Under identical operating conditions, increasing wellbore size or bottomhole pressure differential significantly prolongs the duration of gas invasion into the wellbore. Conversely, elevating kill fluid density or inlet velocity effectively suppresses gas influx and markedly shortens the gas invasion duration. These factors influence displacement efficiency and wellbore pressure dynamics by altering the two-phase flow regime, interface distribution, and migration velocity. This study systematically elucidates the gas–liquid two-phase flow behavior during kill fluid displacement in large-diameter wellbores and quantifies the effects of key parameters. The findings provide a theoretical foundation for optimizing well kill design and well control strategies.