The invention provides a composite beam segment and a pre-assembling method of the composite beam segments. The composite beam segment comprises a pair of edge main beams, multiple parallel diaphragmsconnected between the pair of edge main beams, and multiple longitudinal beams orthogonally erected above the diaphragms and arranged parallel to the edge main beams. The pre-assembling method of thecomposite beam segments includes the steps that a molding bed used for pre-assembling the composite beam segments is manufactured; the pair of edge main beams of each composite beam segment are installed sequentially on the combination side in the side span lateral direction and rigidly fixed to the molding bed; the diaphragms are located and assembled according to diaphragm connecting componentspre-arranged on the edge main beams; the longitudinal beams are located and assembled according to longitudinal beam connecting components pre-arranged on the diaphragms; and the anchor plate structures are matched through tools according to the pre-arranged positions on the edge main beams. The upper air operation difficulty in the bridge location installation process is reduced, the lifting speed is increased, it is guaranteed that site steel beam rods are smoothly installed, an assembly period is shortened, and work efficiency is improved.