The invention belongs to the field of water conservancy and hydropower and particularly relates to a drop sill underflow energy dissipation forced air entrainment and cavitation prevention device which comprises a fan-shaped ventilation cavity; the fan-shaped ventilation cavity is formed at an outlet of a drop sill and is clung to a side wall of a stilling pool; the top of the fan-shaped ventilation cavity is flush with a bottom plate at an outlet of a surface hole spillway face; the front edge of the fan-shaped ventilation cavity is provided with a narrow slit outlet; the upstream side wall of the narrow slit outlet is provided with a deflection sill which is adjacent to the outlet of a floodway; the fan-shaped ventilation cavity is connected with a vent pipeline; the vent pipeline is connected with blasting equipment. By virtue of arranging the forced air entrainment and cavitation prevention device at the combined part of the floodway and the stilling pool, pressure air is supplied to a low-pressure area at a high-speed submerged jet initiation site, and an aerated flow protective layer is formed along a jet shear zone, so that the side wall and bottom plate of the stilling pool are effectively protected from being damaged by cavitation.