In order to reduce the car scraping collision accidents caused by bad “rear view” when reversing, a car reverse anticollision warning system is designed. This system takes AT89S52 single chip as the main control unit, and it adopts the HCSR04 ultrasonic ranging module to realize the noncontact distance measurement. On the basis of matching the hardware, we designed the circuit and software, and carried out the test experiment, analyzed the ranging error. This system detection range can be 2~400 cm, ranging accuracy is up to 3 mm, and it can satisfy the requirement of the early warning of a car when reversing in the narrow space.