The invention discloses a beta-Ni (OH) 2 nanosheet array electrode material growing on large-size foamed nickel in situ and application of the beta-Ni (OH) 2 nanosheet array electrode material. The preparation method of the electrode material comprises the following steps: 1, preparing a phosphoric acid solution; 2, immersing the large-size foamed nickel into a phosphoric acid solution, etching at room temperature, taking out the foamed nickel after etching is completed, and drying to obtain foamed nickel with a Ni3 (PO4) 2 precursor generated on the surface; and 3, at room temperature, soaking the foamed nickel with the Ni3 (PO4) 2 precursor generated on the surface in a KOH solution, after soaking is completed, taking out the foamed nickel, and drying to obtain the beta-Ni (OH) 2 nanosheet array electrode material growing on the large-size foamed nickel in situ. The preparation method of the electrode material is simple and low in energy consumption, and the prepared beta-Ni (OH) 2 electrode material has excellent electrochemical performance on the basis of a large size, and is an electrode material with large size, high loading capacity, large capacity and large-size specific capacitance, so that the electrode material can be used for preparing a large-size high-capacity pseudo capacitor electrode, and used for electrochemical energy storage.