The invention discloses a lightweight flat semi-trailer body, relating to the technical field of manufacturing a flatbed semi-trailer, comprising a bearing shell, a bottom plate and a support beam. The housing is bonded to the lower surface of the bottom plate to form a box structure with the bottom plate, a support beam is arranged in the box-shaped structure and is connected with the shell and the bottom plate, a support beam is arranged in the center of the box structure along the longitudinal direction of the vehicle body, reinforcing structures abutting on both side walls of the housing is disposed on both sides of the support beam in the housing, the reinforcing structures comprise a plurality of support frames arranged along the length direction of the shell, and the support framesare abutted against the two sides of the support beam and the inner wall of the shell. The invention solves the problem of low transverse structural strength of the semi-trailer body, has stronger transverse structural strength, and greatly improves load bearing capacity and robustness.