The utility model provides a semitrailer toolbox which is equipped under the carriage soleplate of the semitrailer which is also equipped with a car frame with a girder, wherein the toolbox comprises a soleplate, a fore wall panel which erects on one side of the soleplate upwardly, a rear wall panel which erects upwardly on the side opposite the fore wall panel, and two lateral wall panels which respectively erect on the left two opposite sides of the soleplate upwardly to form a closed conformation by the fore wall panel and the rear wall panel, wherein the fore wall panel is equipped with at least one toolbox and the height of the rear wall panel is lower than the that of the fore wall panel. The toolbox provided by the utility model has relatively large capacity and is capable of effective utilization the space under the semitraler carriage soleplate and removable connection with the semitraler, thereby being capable of convenient dismantlement of the toolbox without damaging original conformation when users are maintaining toolboxes or cars.