The invention discloses a three-phase boiling bed reactor, which comprises a reactor casing and a three-phase separator, wherein the three-phase separator is positioned at the upper part in the reactor casing and comprises two homocentric round tubes with different inner diameters: an inner tube and an outer tube, the upper ends and the lower ends of the inner tube and the outer tube are all open, and the inner tube and the outer tube respectively comprise a straight tube section and a diffusion section. The three-phase boiling bed reactor is characterized in that the diameter of the straight tube section of the inner tube of the three-phase separator is 0.6 to 0.9 time of the diameter of the reactor casing. Through analyzing the speed distribution rule of liquid in the boiling bed reactor in the radial direction positions, the structure of the three-phase separator is reasonably set, so the operation elasticity of the three-phase separator is increased, and the efficient separation of the three-phase separator is ensured. The reactor is mainly applicable to the chemical reaction taken by different kinds of liquid and gas substances under the condition of being contacted with solid particles, and the reactor has the advantages that the catalyst inventory is large, the utilization rate of the reactor is high, the structure is simple, the operation is easy, and the like.