A trailer of an automatically scanning-type radiation inspecting system used for large-sized object, comprising a trailer body provided with a bevel portion at a tail end of a top surface thereof and positioning recesses adaptive to lower portions of front wheels of a vehicle carrying objects to be inspected respectively, a plurality of pairs of guide wheels mounted to a bottom surface of the trailer body and can be supported and run on rails, connection rods provided at front and rear ends of the bottom surface of the trailer body and used for connecting to wire ropes of winches respectively, anchoring hooks provided at front and rear ends of the bottom surface of the trailer body and used for engaging with wedges arranged on the ground, and holding means provided at front and back sides of the positioning recesses and used for holding the front wheels of the vehicle in the positioning recesses during movement of the trailer. During operation of the trailer of present invention, the trailer pulls the vehicle carrying objects to be inspected to pass through the inspection passage smoothly while the front wheels of the vehicle are held in the positioning recesses on the trailer and rear wheels thereof roll over the ground. The trailer-conveying apparatus formed by the trailer of the present invention and winches occupies less land, being low in cost, simple in structure, reasonable in design, advantageous to shield the radiation, and easy to be maintained.