An original ultrasonic reverse warning system is a new system that can assist drivers while car is braking. It is includes ultrasonic emitter and receiver that can producing and receiving the ultrasonic waves to determine the distance between car and obstacle. But it is not good enough for the safety of cars, in this paper, we are meant to design a system that can help drivers stop the car automatically, an electronic circuit was constructed. According to this circuit we design, a signal was produced to the braking system of car based on the distance between car and obstacle for a safe braking purpose. Error is also discussed and during the experiment, the improvement for the original system has also achieved.