A vehicle backup detector that indicates a distance from an obstruction comprises multiple ultrasonic transducers, a central processing unit, memory, a video camera, a graphics superimposing module and a display device. The ultrasonic transducers emit and receive ultrasonic waves. The central processing unit defines multiple sensing coordinates in a sensing area in which ultrasonic waves are transmitted and calculates a position of the obstruction. The memory is divided into multiple storing fields corresponding to the sensing coordinates to store obstruction data based on the position of the obstruction. A video camera records images behind the vehicle. A graphics superimposing module superimposes graphical images representing ultrasonic waves on the images recorded by the video camera based on the storing fields where no obstruction data are stored. The display device displays images behind the vehicle with the ultrasonic wave images.