The invention relates to the field of copper-nickel (CuNi) alloy corrosion resistance improvement using rare-earth modification and grain boundary engineering, in particular to a preparation method ofyttrium-based heavy-rare-earth copper-nickel alloy. The yttrium-based heavy-rare-earth copper-nickel alloy comprises, by mass percentage, 9-12% of Ni, 1.0-2.0% of Fe, 0.5-1.5% of Mn, less than 0.01%of S, less than 0.01% of P, less than 0.01% of C, 50-300ppm of Y, less than 0.1% of other impurities and the balance copper. The alloy is prepared by smelting the raw materials into casting blanks under vacuum gas protection and subjecting the casting blanks to forging, hot rolling, primary cold rolling, intermediate annealing, secondary cold rolling and finished-product annealing. The high-corrosion-resistant CuNi alloy structurally comprises an alpha-Cu matrix and yttrium-based rare-earth compound inclusion, the grain size (not considering twin crystals) of the matrix is 40-60 micrometers, the low-energy coincidence site lattice grain boundary proportion of the matrix is larger than 75%, and the ratio of maximum free grain boundary length to free grain boundary total length is smaller than 0.3. The CuNi alloy is low in preparation cost, applicable to board and band products, excellent in corrosion resistance and especially applicable to service environments high in seawater corrosionresistance requirements, and the preparation method of the CuNi alloy is simple in process and easy to operate.