The automatic steering system of agricultural machinery equipment includes electric hydraulic automatic steering system and electric steering wheel automatic steering system. However, the electric hydraulic steering system has the disadvantages of high production cost and complex structure, and the electric steering wheel steering system has the problem of small control moment and free travel. In order to solve those problems, an automatic steering mechanism and its electronic control system were designed based on the direct connection between DC motor and full hydraulic steering gear. The automatic steering system mainly consisted of automatic steering actuator, automatic steering controller, hydraulic steering mechanism and wheel angle sensor. Automatic steering actuator and the original car hydraulic steering mechanism was coupled to achieve automatic steering function. In order to improve the accuracy of steering control of the wheels，an automatic steering controller was designed which considered Ackerman angle. An electromagnetic clutch and a steering column torque sensor were installed on the output shaft of the steering drive motor to realize automatic switching between manual driving and automatic driving. The automatic steering system designed was processed and applied to tractor, harvester and plant protection machine, which can meet the needs of use. The test results showed that the average deviation value of wheel response angle was less than 0.1°, the maximum deviation value was 0.158°, the response time of ±20° step signal can reach 1.2s and the overshoot was less than 1%. Moreover, the step response was nonoscillatory, which was faster, accurate and stable than the traditional steering control system. The automatic steering system designed not only met the performance index of automatic steering control, but also reduced the production cost, which provided support for the development of agricultural navigation technology.