1. A method of establishing a plain text document from a HTML document, comprising the steps of: (A) acquiring a HTML document defined by HTML elements, each HTML element composed of tags and content between the tags; (B) pre-processing the HTML document by omitting some of the HTML elements, whereby the rest of the HTML document comprises at least one target tag and at least one corresponding content; (C) using a data structure to store the remaining tags of the pre-processed HTML document; (D) grouping the remaining HTML elements with the remaining tags stored in the data structure of the pre-processed HTML document into at least one target group according to the target tag(s), the step (D) further comprises the steps of: (D- 11 ) sequentially searching for a first content near the target tag from the rest of the HTML document, and identifying the first content as a first base content; (D- 12 ) sequentially searching for next content near the target tag from the first base content, and if there is no next content near the target tag, implementing the step (D- 15 ); (D- 13 ) if an interval between the next content of the step (D- 12 ) and the base content is smaller than a predetermined threshold, identifying the next content of the step (D- 12 ) as a current base content, and repeating the step (D- 12 ), otherwise, implementing the step (D- 14 ); (D- 14 ) grouping the first content and the current base content(s) into a target group, and identifying the next content as another first base content, implementing the step (D- 12 ); and (D- 15 ) grouping the first base content into one of the target groups; and (E) identifying the target group(s) most related to a title of the HTML document by comparing correlation(s) between the target group(s) and the title, and establishing a plain text document having the content of the identified target group.
6. The method of establishing a plain text document from a HTML document as claimed in claim 1 , wherein the predetermined threshold is 1˜5.