In the paper, the research and design work of an automotive safety device is introduced. The device can automatically range, display the distance between the automobile and obstacle and sound alarm at a frequency varying with the distance. Equipped with the device, automobiles can avoid collision caused by drivers not noticing obstacles or not properly estimating the distance between automobile and obstacles when backing up. In the paper, the principle, the software, the hardware and the anti-interference technology of the device are concisely depicted. Test results showed that it is effective in collision avoidance.