Bilayer graphene provides a versatile platform for exploring a variety of intriguing phenomena and shows much promise for applications in electronics, optoelectronics, etc. Controlled growth of large-area bilayer graphene is therefore highly desired yet still suffers from a slow growth rate and poor layer uniformity. Meanwhile, graphene wrinkles, including folds and ripples, form during cooling due to the thermal contraction mismatch between graphene and the metal substrates, and have been far from suppressed or eliminated, especially in bilayer graphene, which would greatly degrade the extraordinary properties of graphene. Here we report the ultrafast growth of wafer-scale fold-free bilayer graphene by chemical vapor deposition. Through well-tuning the alloy thickness and strain regulation of the single-crystal CuNi(111)/sapphire, the full coverage of a 2-inch fold-free bilayer graphene wafer via mainly isothermal segregation has been achieved as fast as 30 s. The tensile-strained CuNi(111) film reduces the thermal contraction mismatch and suppresses the formation of graphene folds during cooling, which is directly observed through in situ optical microscopy. The ultraflat bilayer graphene exhibits wafer-scale uniformity in electrical performance and enhanced mechanical property comparable to the exfoliated ones. Our results offer a promising route for large-scale production of bilayer graphene and enable its various applications.