Disclosed are a method for encoding a video signal and a device therefor. Specifically, a method for decoding an image based on an inter prediction mode comprises: generating a merging candidate list by using a spatial merge candidate and a temporal merge candidate of a current block; obtaining a merge index indicating a certain merge candidate in the merging candidate list; and generating a prediction block of the current block by using motion information of the certain merge candidate, wherein the generating of the merging candidate list may include adding a first merge candidate indicating a collocated block specified by motion information of the spatial merge candidate to the merging candidate list if a reference picture of the spatial merge candidate is same as a predetermined picture.