During drilling operations, formation gas readily invades the wellbore, potentially causing a kick. This study investigated the effects of wellbore size, drilling fluid density, bottomhole pressure difference, and reservoir thickness on gas migration and the gas–liquid two-phase flow during kick development in large-diameter wellbores. A three-dimensional numerical model of a large wellbore was constructed using the VOF multiphase flow model and the SST k–ω turbulence model. Transient simulations were performed to analyze gas–liquid flow patterns under various conditions. Key parameters studied included wellbore size, drilling fluid density, bottomhole pressure difference, and reservoir thickness. Gas–liquid volume fraction contour plots helped interpret the influence of different parameters. Results show that, under otherwise identical conditions, smaller wellbore size, lower drilling fluid density, negative pressure difference (underbalance), and thicker reservoir lead to faster gas migration rates. These conditions also create more complex gas–liquid mixture flow which significantly increase kick risk. Conversely, larger wellbore size, higher density of drilling fluid, positive pressure difference, and thinner reservoir reduce the gas migration velocity. These conditions effectively control gas influx. This research quantifies the impact of the four key factors on annular kick flow in large-diameter wells. It establishes the VOF-SST numerical model. The findings provide theoretical support for optimizing well control parameters in deep wells and preventing blowout accidents.