A parking assistance system includes a camera connected to the ECU module and a plurality of ultrasonic sensors, wherein the ultrasonic sensors are longitudinally arranged on the rear collision-proofbeam of the vehicle, and the camera is arranged on in the middle of the rear collision-proof beam of the vehicle. The ECU module comprises an ultrasonic wave transmitting and receiving control device,a video interface module and a brake control module, wherein the brake control module is connected with a brake actuator, and the ECU module is connected with an on-board display. The distance information relating to the distance between a freight car and a goods unloading platform detected by ultrasonic sensor is fused and displayed on the reversing image. According to the high-precision distance information monitored by radar, the braking system of freight car can be controlled actively to prevent collision, thus assisting the freight car driver to safely and steadily complete the reversingoperation such as unloading and so on.