1. A method for instantly expanding a keyterm, comprising: receiving a query term; according to the query term, performing a data search process by using a plurality of search engines; obtaining a plurality of document snippets related to the query term from each of the search engines, wherein each of the document snippets contains at least a sentence; analyzing each of the document snippets to create a related term table, wherein, for each of the document snippets in the related term table, a plurality of keyterms appeared in each of the document snippet are listed and each of the keyterms possesses a semantic meaning; according to a position relationship between the query term and each of the keyterms in each of the document snippets, performing a correlation calculation for giving each of the keyterms in each of the document snippets a correlation value; performing a semantic merging process to merge the correlation values of the keyterms with the same semantic meaning throughout the document snippets to be a correlation indicator; and according to the correlation indicator for each of the keyterms with the same semantic meaning, determining portions of the keyterms to be a keyterm set of the query term.