1. A method, comprising: ranking structured elements within a structured document, the structured document includes a document element or root element, at least one section element, and at least one paragraph elements, the ranking including: for each paragraph element, in which Weight(t j ,P j ) stands for the weight of the term t i in the paragraph P j , “t f (t i ,P j )” is the term frequency of t i in this paragraph, N denotes the number of documents in the corpus, and n i represents the number of documents containing the term t i calculating the terms' weight according to the calculation: Weight ( t i , P j ) = ln ( 1 + tf ( t i , P j ) ) × ln N n i ; for any section element E j at the upper levels following a bottom-up fashion, in which “I(t i ,E j )” is the entropy measure of the term t i in element E j , wherein if Weight(t i ,E j )≧average(E j )+std_dev(E j ), the term t i is selected as an index term of the element E j and all sub-elements of E j would eliminate t i from their index term list, where “average (E j )” denotes the arithmetic average of all terms' weights in the element E j , and std_dev(E j ) denotes the standard deviation of these weights, calculating term weights using the calculation Weight(t i ,E j )=ln(1+tf(t i ,E j ))×I(t i ,E j ); repeating the calculating the term weights using the calculation Weight(t i ,E j )=ln(1+tf(t i ,E j ))×I(t i ,E j ) until the root element (i.e., the document element) is reached; obtaining paths for all evaluated candidate elements, and assign query terms' weight for elements to paths respectively; ranking paths in which ln N n i is the inverse document frequency (IDF) value of query term t i , which represents the query term's weight and Q is the number of query terms in a query, using the calculation: Rank ( Path p ) = ∑ i = 1 Q Weight ( t i , E j ) × ln N n i ; and returning elements corresponding to the ranked paths in a descending order.