1. A reverse sensing system displaying an overhead vehicle image, for shooting a real scene around a vehicle and generating an overhead vehicle image to be displayed on a screen, so as to assist a driver to reverse the vehicle, the system comprising: a plurality of fisheye lenses, scattered on all sides of the vehicle, for shooting scene images; a trip computer, for reading the scene images, and executing a plurality of built-in image processing procedures to combine the scene images into an overhead vehicle image, wherein the plurality of image processing procedures comprise: a distorted image correction procedure, for reading radially distorted scene images, calculating a central deformation ratio according to a plurality of pixels at central portions of the scene images, and correcting the scene images; a planar transition procedure, for setting a rotation amount according to positions of the fisheye lenses, and rotating the scene images into projection scene images according to the rotation amount; and an image combining procedure, retrieving trusted zones of the projection scene images according to the disposed positions of the fisheye lenses of the projection scene images, so as to combine the images to the overhead vehicle image; and a screen, for displaying the overhead vehicle image when the vehicle is reversed.