Abstract: Abstract: In order to improve the performance of the agricultural machinery automatic navigation system, an automatic navigation path searching method of agricultural machinery based on GNSS (global navigation satellite system) was proposed. According to different demands of farm working, the system could generate the straight line or curve path for agricultural machinery automatic navigation according to users setting. In order to get the straight navigation path, the user should drive the tractor and record the current position marked as point A, and then choose the position marked as Point B at least 10 m far away from Point A. The straight line presupposed navigation path could be obtained by connecting Point A and B and extending the segment AB. The way of obtaining the curve presupposed navigation path is similar to the straight path searching method; the curve fits with several segments, and every segment is analyzed with the straight path searching method. When the navigation task began, the system would compare the current position and heading information of the tractor with the presupposed path to get the lateral deviation and heading deviation. In addition, a pure pursuit mode based on preview points research was proposed for steering control. The method didn't involve the complicated control theory, so that it could adapt the navigation system better. In the aspect of turning control, arcuate turning and pyriform turning patterns were selected as the major research objects. The turning path could be generated by the navigation system according to the tractor working width and the minimum turning radius after the users chose the kind of turning pattern, and a series of points could be chosen according to the tractor speed and each point was evenly spaced. When the navigation task began, the searching radius and preview point should be set according to the speed of the tractor. There were several points on the default navigation path falling in the searching circle; the point with the largest ID (identification) number would be selected as the preview point, and then the path of the tractor arriving to the preview point and the control turning angle would be obtained. To verify the path search method and the model of pure tracking performance, a tractor automatic navigation software was designed and implemented. The industrial computer as the carrier of navigation software, processed the GNSS data, IMU (inertial measurement unit) data and PLC (programmable logic controller) data, and then generated the corresponding decisions. A John Deere tractor was used as the platform for experiments, and the straight line / curve navigation experiments based on GNSS positioning technology were designed. The results of experiments were as follows: In the straight line navigation experiments, when the speed of the tractor was 0.8, 1.0 and 1.2 m/s, the root-mean-square error was 3.79, 4.28 and 5.39 cm respectively; in the turning navigation experiments, when the speed of the tractor was 0.6 m/s, the root-mean-square error of the arcuate turning navigation was 25.23 cm and the root-mean-square error of the pyriform turning navigation was 14.42 cm; for the comparison experiment, the root-mean-square error using the proposed method and fuzzy control method was 4.30 and 5.95 cm respectively in straight line navigation module, and 13.73 and 21.40 cm respectively in curve navigation module. The path searching method and the pure pursuit mode based on the researching of preview points can satisfy the requirement of the farmland works effectively.