The invention discloses an ultra-small size high-power density permanent magnetism external rotor motor device comprising a rotating shaft, the left and right sides of the rotating shaft are respectively connected with a front end cover and a rear end cover through a front bearing and a rear bearing; an aluminum casing with radiating ribs is arranged between the front end cover and the rear end cover, the rotating shaft is connected with a rotor yoke via a shaft elastic baffle ring, the rotor yoke is arranged in a housing formed by the front end cover, the rear end cover, and the aluminum casing with radiating ribs, the rotor yoke is provided with a plurality of reinforcing ribs at intervals to form a local fan blade structure; the rotor yoke is connected with magnetic steel through a magnetic steel frame; the inner side of the rear end cover is connected with a stator core by a fastening screw, and the rotor yoke covers a stator core. The motor device provided has the advantages of compact structure, simple control, simple manufacturing process, low cost, convenient installation and long service life, the reinforcing ribs on the rotor yoke generates the local fan blade structure to timely transfer the internal temperature heat to the aluminum casing with the radiating ribs, and the heat dissipation effect is good.