The invention discloses a device and method for preparing diesel oil by using waste engine oil. The device comprises a catalytic oxidation reactor, an extraction column, a solvent regenerating column, a catalytic cracking reactor, a cracking furnace and a fractionating column, wherein a regeneration circulating pump and a regeneration circulating return pipe are respectively arranged between the solvent regenerating column and the cracking furnace; a tar transfer pump and a first radiation section outlet pipe are respectively arranged between the catalytic cracking reactor and the cracking furnace; and a fractionating column kettle circulating pump and a second radiation section outlet pipe are respectively arranged between the cracking furnace and the fractionating column. The method adopting the device for preparing diesel oil by using waste engine oil comprises the following steps: (a) catalytic oxidation; (b) extraction and desulfurization; (c) catalytic cracking; and (d) fractionation. The sulfur content of the diesel oil prepared by the device and method disclosed by the invention sulfur reaches Europe IV and V standards, the product quality is high, the reaction and equipment investment is little, the problem of environment pollution is solved, the waste recycling is really realized, and social and economical benefits are greatly increased.