Abstract: Vectorization is a key issue in the process of recreating engineering drawings from scanned input image．This paper introduces the technique of adaptive window vectorization based on knowledge base inference．It not only avoids distortion caused by the commonly used thinningbased recognition process at cross point，but also achieves the accurate positioning of cross point． The constraint conditions of adaptive window and the approach of constructing black-run-length structure drawing are also given．Besides，the organization mechanism of knowledge base is analysed in detail．