Abstract: Gas-liquid gravity displacement problems occur frequently during drilling in fractured gas-bearing beds, seriously threatening drilling safety. However, the current research mainly focuses on fractured constant-pressure gas-bearing beds and does not pay much attention to the gas-liquid gravity displacement problem in gas-bearing beds with constant-volume enclosed system. To this end, a visual experimental device for the coupled flow of gas-liquid two-phase fluids in the wellbore and fractures was developed, and simulation experiments of gas-liquid gravity displacement in fractured constant-volume enclosed systems were carried out. The effects of fracture aperture, bottom hole differential pressure, drilling density, and viscosity on gas-liquid gravity displacement were evaluated. The results show that the bottom hole differential pressure, is the dominant factor affecting the gas-liquid flow state, and with the formation energy being released, the bottom hole under-differential pressure is getting smaller and smaller. The well bottom undergoes the transformation from gas invasion to gas-liquid gravity displacement and then to lost circulation. Gas-liquid gravity displacement only occurs within a certain range of differential pressure, and a larger bottom hole under-differential pressure indicates a larger gas invasion rate and a smaller lost circulation rate. Larger fracture aperture and density mean larger gas invasion rate and lost circulation rate. With the increase in the viscosity of the drilling fluid, the gas invasion rate and lost circulation rate decrease significantly. A multi-factor empirical prediction model of gas invasion rate and lost circulation rate was established, and the main controlling factors of gas invasion and lost circulation are bottom hole differential pressure and fracture aperture, respectively; moreover, the prevention and control method of gas-liquid gravity replacement was proposed. The results provide a theoretical basis for the safety control of wellbore flow when drilling a fractured constant-volume enclosed system gas-bearing bed.