Abstract: Based on the etching mechanism of prior austenite grain boundary, the display effect of prior austenite grain boundary in low activation ferrite/martensitic steel was evaluated, and the formulation of etchant, the content of corrosion inhibitor and the etching time were studied. The results show that the martensitic lath and the fine precipitates with different morphologies and sizes of the steel could be etched out in ferric chloride and nitric acid solution, but the display effect of prior austenite grain boundary was not good. Saturated picric acid solution and corrosion inhibitor can etch clear grain boundaries. If there were too few corrosion inhibitor, the etching rate was too high, and it was difficult to control, which could not achieve the effect of inhibiting the imaging of intragranular structure. If there was too much corrosion inhibitor, it was easy to adsorb and form film on the metal surface, and the etching rate was too low, so the corrosion imaging of grain boundary and intragranular structure was inhibited at the same time. In 100 mL saturated picric acid solution and 0.8 g corrosion inhibitor slightly boiling etching for 4 min, the grain boundaries of the steel were clearly identified. Compared with energy spectrum analysis results, the metallographic diagram of optimal etching formulation could identify the grain boundary and grain size more intuitively, and the requirements for sample preparation were lower, and the image acquisition process was more simple and efficient.