The invention discloses high-hardenability large-size wind power bolt steel and a manufacturing method thereof, and belongs to the technical field of high-strength wind power bolt steel. The steel comprises chemical components including, by weight percent, 0.35%-0.45% of C, smaller than or equal to 0.20% of Si, 0.60%-1.00% of Mn, smaller than or equal to 0.012% of P, smaller than or equal to 0.005% of S, 1.00%-1.50% of Cr, 0.15%-0.40% of Mo, 0.0005%-0.003% of B, 0.03%-0.08% of Ti, 0.02%-0.05% of Als, smaller than or equal to 0.005% of N, 0.0001%-0.0003% of Ca and the balance Fe and inevitableimpurities. The high-hardenability large-size wind power bolt steel is mainly used for manufacturing large-size wind power bolts at the 10.9-12.9 level, wherein the dimensions of the large-size wind power bolts are controlled in the range of 42-75 mm. High hardenability is obtained by controlling the content of acid soluble aluminum in the steel, the element upper line and lower line range is calculated and designed through a hardenability model, the section hardness and mechanical performance stability of the bolts are improved, grains are refined, and the low-temperature impact toughness ofthe large-size bolt steel is improved.