The invention belongs to the alloy steel field, particularly relating to steel for a fine grain carburization gear and a preparation method thereof. The steel comprises the following chemical compositions in weight percentage: 0.15 to 0.25 percent of C, less than or equal to 0.35 percent of Si, 0.60 to 0.90 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.010 percent of S, 0.80 to 1.20 percent of Cr, 0.15 to 0.35 percent of Mo, 0.02 to 0.08 percent of Nb, 0.0005 to 0.0035 percent of B, 0.02 to 0.06 percent of Al, 0.01 to 0.04 percent of Ti, less than or equal to 0.015 percent of [N], less than or equal to 0.0015 percent of [O], the balance being Fe and inevitable impurities. Meanwhile, Ti is more than or equal to 2[N] and B is more than or equal to ([N] minus Ti / 3.4) / 1.4 plus 0.001; and the rolling production process with the finish-to-gage temperature below 900 DEG C is adopted. Compared with the prior carburization gear steel 20CrMoH, the grain size of the steel after carburizing and quenching exceeds ten grade, the bending fatigue resistance(sigama-1) is improved by more than 15 percent and the contact fatigue life(L10) is improved by more than 30 percent.