The invention relates to a 100-260 mm-thick tempering-state prehardened plastic mold steel plate. The steel plate has the following chemical components in percentage by mass: 0.35-0.45 of C, 0.20-0.50 of Si, 0.60-1.00 of Mn, 1.00-2.50 of Cr, 0.20-0.50 of Mo, 0.04-0.15 of V+Ti, 0.01-0.04 of Als, 0.015 or less of P, 0.005 or less of S, and the balance of Fe and inevitable impurities; the steel plate is uniform in performance; the overall hardness difference is not more than 2.0 HRC; and the steel plate reaches NB/T47013.3 plate part I grade requirements by ultrasonic detection. The overall hardness difference is not more than 2.0 HRC; the steel plate reaches NB/T47013.3 plate part I grade requirements by ultrasonic detection; and the total inclusion grade is not higher than 2.5. The chemical components of the mold steel plate are simplified; the improvement is performed from the production process angle; the difference of the steel plate in the thickness direction is reduced; and in particular, the section hardness uniformity is improved.