5. The information searching method as claimed in claim 1 , wherein the step of optimizing includes the steps of: selecting, from the preliminary search result, a resulting document sequence having relatively high reliability; arranging the documents of the resulting document sequence in order of creation time; grouping the documents arranged in order of creation time into plural groups; calculating a sum of reliability values of documents of one of the groups to obtain and output a group reliability value, a maximum value and a minimum value of the reliability values of the documents of the one of the groups being regarded as a group action range; adjusting the reliability values of the documents in the resulting document sequence; and arranging the documents in descending order of the adjusted reliability values to obtain optimized results.
5. The information searching method as claimed in claim 1 , wherein the step of optimizing includes the steps of: selecting, from the preliminary search result, a resulting document sequence having relatively high reliability; arranging the documents of the resulting document sequence in order of creation time; grouping the documents arranged in order of creation time into plural groups; calculating a sum of reliability values of documents of one of the groups to obtain and output a group reliability value, a maximum value and a minimum value of the reliability values of the documents of the one of the groups being regarded as a group action range; adjusting the reliability values of the documents in the resulting document sequence; and arranging the documents in descending order of the adjusted reliability values to obtain optimized results. 8. The information searching method as claimed in claim 5 , wherein the step of adjusting includes the step of: determining whether each of the documents in the preliminary search results is in group action ranges of one or more of the groups, and if the reliability value of the document is in the group action range of one of the groups, increasing the reliability value of the document by b*Rg, where b is a parameter for controlling adjustment magnitude of the document creation time on the preliminary search result, a maximum of b is not greater than 5% of an average reliability value of all the documents, and Rg represents the group reliability value of the group.