The axial dispersion coefficient of liquid in downflow slurry bubble column is estimated experimentally by the residence time distribution (RTD) technique. Its variation with superficial gas and slurry velocities, separator pressure, and slurry concentration is analyzed. The axial dispersion coefficient is correlated in terms of dispersion number with the velocity ratio of slurry to gas, Archimedes number, and Reynolds number by dimensional analysis. The mixing time is represented by the isotropic turbulence theory. We found that the axial dispersion coefficient in this present gas-liquid-solid three-phase downflow bubble column is an order of magnitude higher than that in upflow three-phase system. The dispersion coefficient of bubble motion (Db) and the velocity characteristic factor (k) are analyzed based on velocity distribution theory. The present study may be useful for further understanding and developing of specific multiphase reactor in industrial applications.