The utility model relates to a semitrailer girder, including last pterygoid lamina, web and lower pterygoid lamina, the web weld in go up the pterygoid lamina with down between the pterygoid lamina, the web includes narrow cross -section portion and wide cross -section portion, seted up the multiunit group of punching a hole in the wide cross -section portion, every group punches a hole the group and includes three punching a hole, isosceles triangle is arranged into to three punching a hole, every the periphery that punches a hole all has reinforcing frame portion, reinforcing frame portion with narrow cross -section portion is not in the coplanar. The beneficial effects of the utility model are that: through through the trompil on the web to the peripheral punching press of working pressure machine to cutting the hole makes the limit of an outstanding web of all side bands in hole, and not only intensity is not influenced to make semitrailer girder after the shaping, and self weight loss has increased the cargo weight who transports in addition, become axial symmetry or centrosymmetric shape with the shape design of trompil, increase the stability of structure, the width of reinforcing frame portion is unanimous, guarantees that the girder atress is even.