An access method of image data includes storing multiple pixel line of image block into one line of storage space, automatically switching line when storage edge in met and calculating composite address of said image block, continuously storing data of the other image block at blank position of this line storage space if a line space in not fully occupied by data of said image block, picking up data of said image block according to composite address of said image block. Two decoding methods of image data and their devices are also disclosed.