The invention discloses a method for selecting an intraframe prediction mode based on an M algorithm, and a device thereof. The device comprises an intraframe prediction device and a block-mode cost accounting and block reconfiguration device which are connected in sequence. The device of the invention is characterized by also comprising a data management device which is used for storing buffer data of reconstructed images and managing information of adjacent pixels and paths, and a path cost accounting and prediction mode selecting device. The method for selecting the intraframe prediction mode, which is disclosed by the invention, realizes a multi-path global search of the intraframe prediction mode of the block so as to reduce the possibility that the search with the best mode of the intraframe prediction mode of the block falls into the local minimum.