For the case of extracting straight line of image in paper, the definitions of run-length unit, central-run-length unit and maximum-adjacent-search area are given. An algorithm of extracting straight line of image in paper based on extraction and mergence of run-length unit is proposed. The experimental results and analysis of algorithm complexity show that, comparing with methods based on Hough transform, the new method is simple and has excellent performance for extracting straight line of document form.