Automatic steering control plays an important part in an automatic navigation system. A Dongfanghong-X804 tractor is used as a platform and an electro-hydraulic proportional valve is added to the original manually operated steering system. The original circuit of the tractor is improved. An electronic control unit is designed to control the valve, which is composed of a microcomputer (C8051F040), an angle sensor (KMA199), and a CAN bus network. A SD card data storage system is realized for real-time storing experimental data. The experimental results show that the largest and the average signal tracking errors are 1.1° and 0.5°, respectively, and the average delay is 0.2 s. The automatic steering system has good response and can meets the performance requirements.