The invention provides round steel for a gear and a preparation method thereof, and belongs to the technical field of steelmaking. The round steel for the gear mainly comprises the following chemicalcomponents in percentage by mass: 0.18-0.22% of C, 0.80-1.00% of Mn, 1.05-1.15% of Cr, 0.035--0.045% of Ti, 0.0050-0.0070% of N, and the balance Fe. The diameter Di of the round steel for the gear is1.34-2.12 inches; the mass percentages of C, Mn, Cr and Ti are X1%, X2%, X3% and X4% in sequence; and Di is equal to -2.8.8 + 9.3X1 + 1.5X2 + 1.2X3-0.0043.0043X 4. The preparation method of the roundsteel for the gear performs smelting according to the components and the sizes. The round steel for the gear precisely controls the components of Ti and N, and effectively and stably controls the hardenability bandwidth to be not more than 4HRC through Di formula correction.