1. A method for classifying a document including a plurality of feature words, the method comprising steps of: calculating a plurality of similarities between the document and a categorical basic knowledge; calculating a first ratio of a first largest similarity to a second largest similarity of the plurality of similarities; storing the feature words of the document as an extensive categorical knowledge when the first ratio is larger than a first threshold value; and updating the categorical basic knowledge by using the extensive categorical knowledge.
3. A method as claimed in claim 1 , further comprising a step of marking the document into a category of the categorical basic knowledge when the first ratio is larger than the first threshold value.