Disclosed are a method for encoding/decoding an image and a device therefor. Particularly, a method by which a decoding device decodes an image comprises the steps of: deriving a part mode of a coding block within a predefined part mode set; decoding a motion parameter in a prediction block unit partitioned from the coding block according to the part mode; and generating a predicted block by using the motion parameter decoded in the prediction block unit, wherein the part mode set includes an asymmetric 4-block part mode, the asymmetric 4-block part mode enables four prediction blocks to be partitioned from the coding block, and a horizontal partition and/or a vertical partition can be defined as an asymmetric partition form.