Through the rapid direct-reading spectrometer and in situ analysis of comparative analysis of the working principle, as acid etching can not quantify the segregation in slab, slab was cut into small specimens ,which were detected by fast direct reading spectrometer, simulating in situ analysis. The result from direct reading spectrometer can describe the segregation in slab quantitatively and trends same as acid etching. The fast spectrum analysis to quantify the segregation of slab provides a test methods for slab and substitute.