—— Automatic Round-trip Car Abstract Since 1970s the SCM appeared, because of its extremely high performance-price ratio, it suffered recognition and concern by people. It is widely used and fast development. 51 series SCM is the most typical and most representative of all the SCMs. This design uses AT89S51 as core. With necessary peripheral circuit, design a intelligent automatic round-trip car. It is provide 5V Dc power. In the hardware, in addition to CPU, to drive the car running is H bridge circuit. Through the infrared sensors, it judges ground state and conveys the signal to CPU then conduct calculation. Program uses C language. The whole system can accomplish car forward and reverse operation and has regular backing, and other functions.