At present, deep and ultra-deep formations are recognized as crucial domains for increasing oil and gas reserves and production in China. Oil-based drilling fluid is conventionally employed in ultra-deep well drilling. Multiphase flow behavior in the wellbore is complicated by gas invasion because gas is dissolved and precipitated in oil-based drilling fluids. To address this issue, multiphase flow behavior in the wellbore during gas invasion in ultra-deep drilling with oil-based mud was investigated. A transient gas-liquid two-phase flow model for oil-based drilling fluid in the annulus was established based on the drift-flux model. The effects of gas dissolution, gas precipitation, and high-temperature/high-pressure conditions were considered. A second-order AUSMV (advection upstream splitting method V) scheme was constructed by applying the Van Leer limiter, and the model was solved. Variations in overflow volume, annular gas holdup, and wellbore pressure during gas invasion in ultra-deep wells were investigated using the established model. The results indicate that a larger initial pressure difference causes a larger decrease in bottomhole pressure, a larger overflow volume, and a deeper gas precipitation point. The higher drilling fluid flow rate increases liquid velocity, shortens gas migration time to the wellhead, reduces the decrease in bottomhole pressure, decreases overflow volume, and raises the gas precipitation point. A theoretical basis for accurate prediction of bottomhole pressure and safe, efficient management of gas invasion in ultra-deep drilling is provided.