A metallographic corrosion method for displaying a GCr15 original austenitic grain boundary comprises adding picric acid 5g into distilled water100ml and mixing continuously, then adding sodium dodecyl benzene sulfonate 5ml 50% and mixing, finally adding ferric chloride 2g, and using after placing for 24 hours. A sample is grinded roughly, grinded finely, polished, cleared, dried, immersed into caustic erodent for 2-5 minutes according to normal method under a quenching tempering condition until etched surface is changed into silver grey, cleaned up through flowing water, cleaned with alcohol 95%, and dried. If the sample is over-corroded, polishing paste W0.5-1.0 or metallographic polishing egent0.5-1.0 is added on silk polishing cloth, the sample is polished slightly with hands, then cleaned with alcohol 95%, and dried. According to practical condition, grain granularity measurement can adopt methods of picture contrast, grid, intercept, quantitative metallography, and the like, to assess according to relevant standards.