Disclosed is an asymmetric co-rotating multi-screw extrusion device, an extruder, and a processing method thereof. The barrel is provided with a feed port located at one end of the screw mechanism and a discharge port located at the other end of the screw mechanism, and the barrel comprises a conveying section, a melting section, an exhaust section, and a mixing extrusion section sequentially arranged from a side thereof where the feed port is located, wherein the exhaust section is provided with an exhaust hole. The screw mechanism comprises at least a first screw and a second screw arranged parallel to the first screw, wherein at least one of the first screw and the second screw has a cross-sectional profile comprising an arc forming a radial step between a root and a crest thereof, and the first screw and the second screw are asymmetrically inter-meshed and co-rotated at the same speed.