In order to specify the ripple of the large reflecting optical item in different scale, the algorithm based on the structure function was investigated. Firstly, the concrete steps of the algorithm were presented. Secondly, the error composition character was discussed in theory. Furthermore, the method proposed here was applied to the mirror figure data of a 1.23 meter mirror, making the conclusion that the simplification was correct and application was feasible. Lastly, the application to the thirty meter telescope tertiary mirror was processed, the ripple of the mirror figure in different levels was specified. This work will guide the way to the large flat mirror specification.