1. A document processing method, comprising the steps of: (a) obtaining a document, dividing the document into a plurality of terms, and giving a syntactic index to each of the terms; (b) dividing the document into a plurality of sentences according to at least one type of punctuation marks contained in the document; (c) selecting a plurality of key syntactic indexes from the syntactic indexes, transforming each of the sentences into a preliminary key terms sequence according to the key syntactic indexes, wherein each preliminary key terms sequence includes at least one preliminary key term corresponding to the key syntactic indexes; (d) selectively expanding the at least one preliminary key term of a representative sentence of the document into an advanced key term; and (e) determining at least one core candidate sentence from the plurality of sentences according to the preliminary key term and the advanced key term to form a core abstract of the document.