1. A method for decoding a video signal comprising: deriving, based on a position and a size of a first prediction block, at least one merging candidate relating to the first prediction block, wherein when a size of a block indicated by parallel merge level information is equal to or greater than a first size and a size of a coding block which comprising the first prediction unit is equal to a second size, the position of the first prediction block is set equal to a position of the coding block and the size of the first prediction block is set equal to a size of the coding block; generating a merging candidate list for the first prediction block based on the at least one merging candidate; determining motion prediction information of the first prediction unit based on the merging candidate list, the motion prediction information comprising a motion vector and a reference picture index; and obtaining prediction samples of the first prediction unit based on the motion vector and a reference picture of the first prediction block, the reference picture of the first prediction block being determined by the reference picture index.