The invention discloses an overall-length cast tube roundness corrector which is of simple structure, skillful design and reasonable layout. A special mechanism, which is provided with a press roll having the same length with a cast tube, is designed for solving the problems of conventional roundness correcting equipment in use, and realizes online overall-length rolling type cast tube roundness correction. Besides, by the overall-length cast tube roundness corrector, roundness of the cast tube can be corrected while straightness thereof can also be corrected. The overall-length cast tube roundness corrector has excellent roundness correction effect, the corrected cast tube is not easy to rebound, and its production efficiency is increased greatly as compared with that of the conventionalequipment. Without affecting production of the whole production line, the corrector can be used as a rail for sending the cast tube into the next station directly when roundness correction is not needed. Therefore, the overall-length cast tube roundness corrector has various advantages, has wide marketing prospect, and is particularly suitable to popularize and apply in the field.