Abstract: In deep and ultra-deep oil and gas wells, abnormal pressure is often encountered while drilling due to their complex casing program and larger borehole sizes, which results in a narrowed safe operating window. When gas intrusion occurs, a gas-liquid two-phase flow forms in the annulus of the wellbore. Conventional well killing methods based on flow pattern transition theories of conventional annulus sizes are prone to exceeding this narrow window, leading to alternating influx and loss and therefore the optimal well killing timing is missed. To address this issue, a numerical simulation method for gas-liquid two-phase flow in large-size annulus was developed using the volume of fluid (VOF) model and was verified with literature data for accuracy. In the simulation of gas-liquid two-phase flow in an annulus with a hydraulic equivalent diameter of 196.8 mm, four flow patterns including bubble flow, cap bubble flow, slug flow, and churn flow were identified and analyzed. A gas-liquid flow pattern map was created, and criteria for flow pattern transitions were established, revealing the influence of annulus size on flow pattern transitions. The results indicate that compared with conventional annulus size, the range of bubble flow expands in larger annuli, with a transitional flow pattern, namely cap bubble flow occurring between bubble and slug flows. The boundaries for flow pattern transitions shift to the right to some certain degree. In conventional annulus size, bubble coalescence and the formation of Taylor bubbles are common, making well killing operations more challenging. Consequently, well control parameters designed for large annuli tend to be bigger. On the contrary, the well control parameters designed based on the new criteria meet the requirements of well killing better in narrow windows and large borehole sizes, thereby improving the efficiency and safety of well killing operations.