Abstract: The principle of analyzing inclusions in steel by spectrum instrument was presented. The curve of Al₂O₃ pulse intensity by spectrum with its content by chemical analysis was established based on studying original Al₂O₃ pulse intensity from spectrum instrument, the relationship between the spectrum intensity and the inclusion size was determined, at last Al₂O₃ content and size distribution analysis on-line was realized. In comparison of chemical methods, the average error of the spectrum method is 3×10^-6. The time of analyzing the inclusion content and size was shortened to 5 min from 2-5 d, which can offer a effective inclusion analysis technique for inclusion on-line control in steel-making and refinement processes.