The invention relates to a pure aluminum grain size corrosive agent as well as a preparation method and a corrosion showing method thereof. The pure aluminum grain size corrosive agent consists of the following components: 5 to 7 g of K2CrO4, 6 to 10 ml of nitric acid, 1ml of a saturated picric acid water solution, 0.4 to 0.6 ml of hydrofluoric acid, 4 to 8 ml of hydrochloric acid and 18 to 22 ml of water. A pure aluminum matrix is a simplex alpha solid solution, different phases exist in the alpha solid solution, and a grain boundary is generated after corrosion; after adopting an existing corrosive agent and an existing corrosion method, a pure aluminum crystal boundary difficultly has obvious level differences and is poor in corrosion effect. The pure aluminum grain size corrosive agent provided by the invention is formed by compounding the nitric acid, the hydrofluoric acid, the hydrochloric acid, potassium chromate and picric acid; and with a suitable concentration, the K2CrO4 can simultaneously play the effects of an oxidant and a mordant, can corrode the crystal boundary, can also colorize alpha grains of different phases and can make the crystal boundary be more obvious and clear in cooperation with the effect of the picric acid.