In order to solve the problem of the low efficiency caused by the traditional query expansion retrieval methods，a novel approach based on extension knowledge was issued with a novel rhomb reasoning method by using extension theory. By introducing the extension relationship between concepts with extension reasoning, the proposed method can relationally solve the detection problem of sensitive information. For illustration, an example was utilized to show the feasibility of the method in solving detection problem with concept query expansion method. Empirical results showed that the proposed method has a good performance on detection of short text which has a low degree of description, and could reduce the omission of relative information. The proposed method can improve the detection precision and recall of the document sensitive information．
