Intelligent robot system has become an essential development direction for managing a farm in the whole-process, all-day, and unmanned environment in smart agriculture. Therefore, it is necessary to cooperate with the harvester and grain truck to realize the autonomous operation in the harvest link. In this study, a longitudinal relative position cooperative control system was designed in the process of master-slave navigation harvesting and co-unloading grain, suitable for the trailer drive system with high nonlinearity. A parallel cooperative model of two machines was established to calculate the deviation of longitudinal relative position, where the relative position of harvester and grain truck was geometrically represented. A linear tracking was also utilized to control the transverse distance deviation, due to the fact that the harvester and grain truck separately planned the operation path. In longitudinal distance error, the throttle of the grain truck was used to adjust the longitudinal relative distance and further control the forward speed. A position-velocity coupling controller was designed to calculate the desired throttle, including a speed feedback Proportional Derivative(PD) controller and a position-velocity integrated decision bang-bang controller. The switch function of the bang-bang controller was derived from the dynamic features with good robustness. An open-loop second-order transfer function of throttle speed was generated from area identification to optimize the parameters of the controller. A simulated model of longitudinal relative position control was constructed to optimize the parameters of position-velocity coupling controller, according to the transfer function. A field experiment was conducted to verify the reliability of the model. Additionally, a comparison was also performed on the designed control system and traditional PD control. The simulation results showed that the designed control was fully adapted to the change of host speeds in practical operation, indicating better adaptability than the traditional PD. A two-machine cooperative navigation test was set to determine the adaptability and accuracy of longitudinal relative position control of position-velocity coupling in field operation. Both the harvester (Lovol Heavy Industry GE80S-H) and grain truck (Lovol Heavy Industry M1104) were installed on an electrically controlled chassis, to realize electronic steering and speed control of engines. Real-time kinematic and global navigation satellite systems (RTK-GNSS, K728 of Si Nan Company) were used to locate modules, with the location acquisition frequency of 10 Hz, and the accuracy of horizontal positioning ± (10+D×10-6) mm, where D is the distance between the base station and the mobile station, km. A wheel corner sensor (BEI-9902120CW) was used with the nonlinearity of ±2%, and A/D sampling accuracy of 12 bits. The switch actuator was Rexroth HT801053. Two sets of communication modules with 2.4 GHz frequency were used for the dual-machine communication (EBYTE company E34-DTU (2G4D20)), where module and control terminal were communicated via RS-232, and the control terminals were AGCS-I controllers with touch screens. The CAN bus was adopted to connect the control terminal with the chassis electronic control unit of the dual machine. This position-velocity coupling longitudinal relative position control was transplanted into the AGCS-I controller. Metrowerks CodeWarrior was adopted for ARM Developer Suite v1.2 development. Collaborative system experiments were conducted in a pilot field at the Lovol Arbos Intelligent Agriculture Demonstration Base. The experiment result showed that the longitudinal relative position deviation converged rapidly under the initial longitudinal deviation of 3, 7, and 10 m when the speed of the main engine was 1 m/s. The average adjustment time of system response was 7.73, 17.2, and 23.2 s, respectively. The average steady-state longitudinal relative position deviation was 0.091 8 m, and the standard deviation of steady-state longitudinal relative position deviation was 0 m, while the control accuracy of 1 173 suitable for the requirement of co-unloading grain, indicating excellent initial deviation adaptability. In addition, a wheat harvest test of the dual-machine cooperative system was carried out in Jinchang, Gansu Province of China. The performance of longitudinal relative position control with position-velocity coupling was obtained in the actual harvest operation. The field experimental results showed that the average steady-state longitudinal relative position deviation was 0.077 8 m, and the standard deviation of steady-state longitudinal relative position deviation was 0.091 3 m, indicating high cooperative accuracy in the need of harvest cooperative grain unloading. The finding can provide sound support for the high-precision independent system of harvest operation in smart farming.