Abstract: In a method for catalytic hydrodesulfurization of a heavy petroleum hydrocarbon oil in a reactor composed of a cylindrical pressure vessel and a cylindrical inner pipe, in which hydrogen gas, oil liquid and solid catalyst particles are concurrently present and the catalyst particles are fluidized and recycled in the reactor without flowing out of the reactor, an improvement for the prevention of the catalyst's efflux is achieved by the provision of a bubble-collecting plate and at least two catalyst particlescatching plates above the inner pipe in the reactor.