Abstract: In order to improve the accuracy of the calculation results of the gas-liquid two-phase flow along the bottom hole after gas cut, the diameter distribution characteristics of initial bubbles at the bottom hole after gas cut were experimentally studied, and a prediction model for the average diameter of the initial bubbles was established. Xanthan gum solutions with different mass fractions were used to simulate drilling fluids, and porous media were utilized to simulate the formations. The bottom hole diameter distribution characteristics of the bubble groups were observed experimentally under different liquid phase rheology, average pore diameter of formation, and gas cut rates. The experimental results showed that a larger yield strength of the simulated drilling fluid and a higher gas cut rate resulted in a larger distribution of diameter range of the initial bubbles generated. In addition, the diameter of bubbles with the highest frequency of occurences and that of the biggest bubbles both increased. However, the formation’s pore diameter had no obvious effect on the initial bubble diameter. According to the experimental results, an experimental prediction model for the average diameter of the initial bubbles invading the bottom hole was established, which comprehensively considered the influence factors such as drilling fluid viscosity, gas flow rate, and surface tension. Furthermore, an prediction model for the average diameter of the initial bubbles invading the well bottom was established by considering the influence of the radial intrusion characteristics of the gas at the borehole wall and the hole deviation angle during the actual drilling. The establishment of the prediction model for the diameter of the initial bubbles at the bottom hole provides theoretical support for the accurate calculation of the gas-liquid two-phase flow along the wellbore after gas cut.