Structured documents, especially the XML documents, are made up of a few logical components, such as title, sections, subsections and paragraphs. The components in each structured document can be represented by an ordered tree model, which can also be viewed as a hierarchical concept relationship. To meet the user's requirements for more precise and concentrated search results, the retrieval techniques should allow the user to retrieve document components with varying granularity. To enable this, suitable querying strategies, especially some special indexing techniques must be developed. This paper put forward an enhancive indexing technique for approximate search by taking advantage of the natural hierarchical structure in text documents. Experiments show that improved performance can be obtained by using such a kind of enhancive indexing technique.