The invention relates to a preparation method of water-soluble nickel hydroxide flaky nano-crystal. The preparation method comprises the following steps: dissolving soluble nickel salt into de-ionizedwater to form a solution A; dissolving soluble citrate into the de-ionized water to form a solution B; dissolving soluble alkali into the de-ionized water to form a solution C; dropwise adding the solution B into the solution A and stirring to form a mixed solution D; dropwise adding the solution C into the mixed solution D and stirring to form a mixed solution E; enabling the mixed solution E toreact for 6h to 24h in a sealed hydrothermal reactor at 120 DEG C to 200 DEG C; cooling to room temperature to obtain a reaction solution; alternately centrifuging and washing sediment obtained by centrifuging the reaction solution with the de-ionized water and absolute ethyl alcohol respectively; re-dispersing the sediment into the de-ionized water and regulating the pH (Potential of Hydrogen) value to 8.0 to 10.0, so as to obtain the water-soluble nickel hydroxide flaky nano-crystal. The preparation method provided by the invention has the advantages of toxin-free and green raw materials, abundant and easy-to-obtain sources, low cost and simple synthesis technology; a product has stable quality and the technical repeatability is good; the water-soluble nickel hydroxide flaky nano-crystal has good crystallization degree and colloid stability and can be used in the fields including electrode materials and catalytic materials.