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An agricultural automatic navigation system was designed on Dongfanghong X-804 tractor to improve the navigation control of agricultural machinery by using RTK-DGPS and double closed-loop steering control. The makeup of the whole navigation system and working principle were presented, among which their main features were: RTK-DGPS could offer positioning data, including heading, roll and pitch parameters, which were acquired from AHRS500GA, and the electro-hydraulic steering controller was developed for automatic steering control. Then, the system control strategy was analyzed and the control transfer function model was developed for trajectory tracking, with a double closed-loop control algorithm for steering system designed according to characteristic of the system nonlinear. The implementation description on an ARM9E-based embedded control system was provided in terms of electronics hardware design. Tests were conducted to examine the navigation system, including a straight line driving test on uneven road, which was to verify the effectiveness of the correction model. The test results showed that the proposed positioning and orientation evaluation algorithm could eliminate the effects of uneven field condition on GPS positioning and the average error of GPS positioning was reduced to 0.43°. Then a test of steering control system was carried out to verify the performance of double closed-loop control algorithm. Test results showed that the steering control system solved the control overshoot well and the average error was 0.40°. Finally, the field test results showed that the performance of automatic navigation system was improved, with average route tacking error was less than 0.019m, average steering angle tracking error was 0.43° and standard deviation was less than 0.041m. The field test results indicated that the proposed positioning evaluation algorithm and double closedloop steering control algorithm on uneven field were appropriated to Dongfanghong X-804 tractor. 