The invention discloses a cross longitudinal movement mechanism applied to a bridge erecting machine, and belongs to the engineering machinery field. The mechanism comprises racks, driving componentsand driven roller wheel components, wherein each rack is fixed on a supporting leg through a hinge pin which penetrates through an upper hinge pin hole of each rack; the lower part of each rack is provided with driving shaft installation holes, and a driving shaft is rotatably connected in each driving shaft installation hole; each driving component is fixed on the outer side wall of each rack, and the output shaft of each driving component is fixedly connected with each driving shaft; each driven roller wheel component is arranged on the inner side wall of each rack through each hinge shaft component; in addition, the driven roller wheel components can rotate by corresponding to the axial lines of the hinge shafts and are meshed with gears sheathed to the outer rings of driving shafts toenable the driven roller wheel components and the driving shafts to be in transmission connection; and a roller shaft driving way is adopted to realize the longitudinal movement of the main girder ofthe bridge erecting machine, the main girder and the longitudinal movement mechanism are in rolling friction but not sliding friction, so that requirements on synchronism are low, and a risk of supporting leg overturning is not in the presence.