The invention relates to carburizing alloy steel, and a preparation method thereof. The alloy of the carburizing alloy steel comprises, by mass, 0.15%-0.25% of C, less than or equal to 0.30% of Si, 0.30%-1.20% of Mn, less than or equal to 0.015% of P, less than or equal to 0.010% of S, 0.30%-1.50% of Cr, 2.80%-3.80% of Ni, 0.30%-1.00% of Mo, 0.01%-0.08% of Al, at least one selected from 0.05%-0.15% of V and 0.025-0.10% of Nb, and the balance being Fe and inevitable impurities. A mass percentage of V, Nb and Al meets an equation of V+Nb+Al <= 0.30%. The carburizing alloy steel provided by the invention has good anti-fatigue performance, excellent comprehensive properties, good hardenability, high yield ratio, extremely small internal oxidation and the characteristic of resisting grain growth at a high temperature, and is suitable for manufacturing heavy-duty gears.