The invention discloses a corrosive for displaying a grain boundary of original austenite of tempered martensite and a corroding method. The corroding method comprises the following steps: preparation of a tempered martensite corroding test sample: obtaining the tempered martensite corroding test sample by a conventional method; preparation of the corrosive: putting 1 gram of picric acid into 100 ml of distilled water, adding b ml of a washing agent after the picric acid is completely dissolved, fully stirring, and stewing under normal temperature; corrosion: putting the tempered martensite corroding test sample into the corrosive, corroding the test sample for T time, taking out the test sample, washing with running water, and cleaning and drying the test sample with alcohol; grain judgment: observing under a microscope of 500 times to obtain a grain displaying picture, and performing grain judgment. According to the corroding method, the test sample is corroded under the normal temperature without heating, so that the method is economical; furthermore, the grain boundary of the corroded tempered martensite is clear and is not affected by tissues, so that respective grains can be observed clearly; the grain judgment can be performed by a comparison method, a grid method and a resection method according to an actual condition, so that the accuracy and the precision are higher.