1. A method for automatically generating an image description, comprising following steps of: obtaining a first image data; analyzing a text file corresponding to the first image data; calculating occurrences and distribution ratios of terms, each of which contains a target word, from the text file so as to obtain a plurality of the terms having the distribution ratios greater than a threshold; comparing the distribution ratios of the terms so as to find out at least a key term; finding out one of a plurality of lexical chains containing the key term with the greatest distribution ratio so as to generate a narrative language chain; and setting the narrative language chain as a description of the first image data.