In order to make the car can avoid obstacles effectively in reverse, this paper introduces a kind of vehicle reversing obstacle avoidance system based on STC90C51 microcontroller ultrasonic distance measurement system in detail.The system could measure distance with the time between transmit wave and reflected wave, and we can use this distance to represent the distance between the trail and the obstacle.If the distance is too close to the voice and buzzer warning, after the actual verification, verify the correctness of the system, when it is applied to reverse the car, the car will be effective to avoid obstacles, to achieve safe driving.