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.