1. An automatic abstract determination method of a document clustering, comprising: (A) forming a database, for correspondingly packing and storing each document clustering including a plurality of abstract contents and body contents of subdocuments preloaded by a user as a document clustering process record file, wherein each document clustering process record file records the abstract contents and the body contents of the subdocuments, and sets a document accumulation counter value, an abstract making accumulation counter value, and a success abstract making counter value; (B) loading the corresponding document clustering process record file according to the document clustering wished to be browsed by the user; (C) loading the abstract contents of one of the subdocuments, refreshing the document accumulation counter value, and determining whether or not the document accumulation counter value is greater than a first threshold value; if the document accumulation counter value is smaller than the first threshold value, executing a step (F); (D) if the document accumulation counter value is greater than the first threshold value, calculating the abstract making accumulation counter value and the success abstract making counter value for generating an abstract making success rate; (E) determining whether or not the abstract making success rate is greater than a second threshold value; if the abstract making success rate is smaller than the second threshold value, executing a step (J); (F) determining whether or not the length of the abstract contents is greater than a third threshold value; if the length of the abstract contents is greater than the third threshold value, executing the step (J); (G) if the length of the abstract contents is smaller than the third threshold value, executing an abstract making process to the body contents of the subdocument, for generating a new abstract content and refreshing the abstract making accumulation counter value; (H) refreshing the new abstract contents to the document clustering process record file for replacing the abstract contents of the subdocument; (I) refreshing the success abstract making counter value; (J) outputting the abstract contents to a display terminal; and (K) repeating the steps (C) to (K), until the abstract contents of all subdocuments in the document clustering process record file are processed.