The invention discloses an etchant for displaying a 690 nickel-base alloy deposited metal grain boundary and an etching method thereof. The etchant comprises ferric chloride, EDTA, deionized water andconcentrated hydrochloric acid. The surface of the 690 nickel-based alloy deposited metal surface is successively wiped with the etchant and a nitric acid ethanol solution. Thus, it is possible to intuitively observe that the surface of a sample changes from bright to frosty during an etching process, and the weld bead can be clearly observed by naked eyes in order to better control the degree ofcorrosion, completely and clearly show the grain boundaries of the deposited metal sample grains, and avoid the covering by corrosion products. The etched different grains have different shades due to different orientations. The columnar crystal structure can be clearly observed and the grain boundaries are clear and complete. The etchant is made of simple raw materials, nontoxic, environmentallyfriendly, convenient in liquid waste disposal, low in cost, and stable and easy to control in a reaction process. The etching method is simple and easy to use, and provides detection basis for the process quality control, the process plan and the quality analysis of nuclear welding deposited layer, and has very important practical significance.