The invention discloses a cast steel back-up roller and a preparation method thereof, and belongs to the technical field of steel rolling rollers. High alloy steel taken as a working layer material of a roller and graphitic steel taken as a core material of the roller are completely metallurgically bonded to each other. The high alloy steel material comprises the following chemical components in percent by weight: 0.4 to 0.6 percent of C, 0.4 to 0.6 percent of Si, 0.5 to 0.7 percent of Mn, 4.5 to 5.5 percent of Cr, 1.0 to 1.2 percent of Ni, 0.6 to 0.8 percent of Mo, 0.03 to 0.08 percent of Re, less than or equal to 0.35 percent of P, less than or equal to 0.030 percent of S and the balance of Fe; and the graphitic steel material comprises the following chemical components in percentage by weight: 1.4 to 1.6 percent of C, 1.7 to 1.9 percent of Si, 0.4 to 0.6 percent of Mn, 0.5 to 0.7 percent of Ni, less than 0.2 percent of Cr, less than 0.2 percent of Mo, less than 0.035 percent of P, less than 0.030 percent of S, and the balance of Fe. The cast steel back-up roller and the preparation method thereof have the advantages that: the online service life of the roller and the operating rate of a rolling mill are greatly improved; simultaneously, the serious consequence of scrap caused by abnormal nodularizing and poor bonding of the roller due to high-temperature tapping and high-temperature casting of the molten iron of nodular cast iron is avoided; and the manufacturing cost of the roller is reduced.