The invention belongs to the technical field of steel bridge structure manufacturing, and discloses a mechanical straightening method for a U rib plate unit. A U rib plate unit mechanical rolling andstraightening machine carries out the following steps of setting the press amount of a pressure head oil cylinder above and straightening machine traveling speed according to plate thickness of a U rib plate unit; straightening deformation of two sides of a U rib in the middle of a U rib plate; adopting the U rib in the middle as a starting point, and from the near to the distant, straightening deformation of two sides of two U ribs symmetrical to the U rib in the middle at the same time; detecting a deformation situation of the U rib plate, and repeating the steps to carry out straightening operation if the requirement is not met; when the number of the U ribs on the U rib plate is an odd number, adopting a group of pressing heads to straighten the deformation of the two sides of the U rib in the middle of the U rib plate; and when the number of the U ribs on the U rib plate is an even number, adopting two groups of pressing heads to straighten the deformation of the two sides of thetwo U ribs in the middle of the U rib plate at the same time. The invention provides the straightening method, which is beneficial to improving the straightening efficiency and the straightening accuracy, and reducing the influence on the strength of the U rib plate.