1. A method for automatically adding a tag to a document, comprising: determining, by an apparatus comprising a processor, a plurality of candidate tag words corresponding to the document; determining, by the apparatus, a corpus comprising a plurality of texts; selecting, by the apparatus, commonly-used words from the corpus as characteristic words; determining, by the apparatus, for each of the characteristic words and each of the candidate tag words, a probability for co-occurrence of the candidate tag word with the characteristic word; abstracting, by the apparatus, characteristic words from the document; calculating, by the apparatus, a weight for each of the abstracted characteristic words; calculating, by the apparatus, in the corpus, a weighted probability for co-occurrence of each of the candidate tag words with all of the characteristic words abstracted from the document; and selecting, by the apparatus, the candidate tag word with a high weighted co-occurrence probability as a tag word to be added to the document; wherein the weight for the characteristic word Y abstracted from the document is denoted as W y , and W Y is equal to a product of the number of times that Y occurs in the document and the number of the texts in the corpus in which Y occurs.