The invention discloses an inlet diffuser of a hydrogenation reactor. The inlet diffuser comprises an inlet cone, a mounting plate and an injection cover. The inlet cone is a conical table-shaped body, rotational flow pipes are uniformly distributed on the upper surface of the inlet cone, the upper part of each rotational flow pipe is of an inverted conical closing-in structure, the lower part ofeach rotational flow pipe is a guide pipe for guiding, an outlet of each guide pipe is fixed on the lower surface of the inlet cone, and a rotary guide vane is arranged in each inverted conical closing-in structure. The mounting plate is a circular ring plate, and the inner ring of the mounting plate is welded and fixed to the outer wall of the inlet cone. The injection cover is a spherical surface or an ellipsoidal surface, injection holes are evenly distributed in the injection cover, and the injection cover is fixedly installed on the lower portion of the inlet cone. Gas and liquid can be fully mixed, the spraying area is large, bias flow of the liquid phase is effectively avoided, initial distribution of oil gas entering a reactor can be greatly improved, the reaction operation condition is improved, and the product quality is improved.