The invention relates to an application of water concentrated with impurity salt, used as crystal growth substance for decomposition of sodium aluminate solution, which applies to the decomposition process of sodium aluminate solution during alumina production. The substance is characterized in that the crystal growth substance is concentrated water serving as a byproduct produced through reverse osmosis in a natural water softening process and has electrical conductivity between 1,500 and 3,000 mu s/cm. The crystal growth substance for decomposition of sodium aluminate solution reacts with sodium aluminate solution to form floc which is large in specific surface area and strong in adsorption capacity, adsorbs and decomposes aluminum hydroxide particles in slurry and allows aluminum hydroxide to agglomerate and grow up. The concentrated water serving as the byproduct produced through reverse osmosis in the natural water softening process can be a byproduct of the prior chemical production. Due to the low cost of the concentrated water serving as the byproduct produced through reverse osmosis in the natural water softening process, the cost for domestic or imported crystal growth substances added in the decomposition of sodium aluminate solution in the prior art is reduced; benefit is increased; and the granularity of the product is improved.