Abstract: Agricultural and rural modernization has been ever accelerating with the promotion of rural revitalization strategy. The transformation of agriculture to intelligence, precision, and environmental protection has been the main trend of development in recent years. Among them, autonomous navigation of unmanned agricultural machinery has drawn much attention to effectively improve accuracy and efficiency. Path tracking control is one of the key technologies in the autonomous navigation of agricultural machinery. A lot of research has been done under relatively ideal working conditions. However, agricultural machinery has to operate in complex and various soil conditions, as well as highly unstructured farmland environments, such as muddy paddy fields, rugged slopes, and slippery grasslands. Agricultural machinery can inevitably encounter various uncertain disturbances during operation in these scenarios, leading to the low accuracy of path tracking and the stability of the control system. Therefore, anti-disturbance and strong self-adaptability are required in the autonomous navigation control of agricultural machinery. This study aims to improve the path following control accuracy and anti-disturbance ability of four-wheel self-steering (4WSS) electrically driven unmanned agricultural machinery in complex farmland environments. A control system of non-singular fast adaptive terminal sliding mode was also proposed using a nonlinear disturbance observer. Firstly, a 4WSS chassis extended kinematic tracking error model of unmanned agricultural machinery was established to consider the influence of drive wheel slip on the chassis motion in the farmland environment, including unknown sideslip speed and steering slip angle. Secondly, a nonlinear disturbance observer was designed to estimate the external lumped disturbance with unknown slip parameters, according to the chassis extended kinematic tracking error model. The disturbance compensation was introduced into the controller. A nonlinear adaptive sliding mode was also constructed to slow down the jitter problem of the nonsingular fast terminal sliding mode, according to the exponential approximation. The controller outputs confirmed the acceptable smooth control amount of the steering actuator. The stability of the composite controller was verified at the same time. Once the 4WSS unmanned agricultural machine tracked the U-shaped target path at 0.4 and 0.6 m/s, the standard deviation in the lateral error of the target operation path was reduced to 8.1 and 9.2 cm at different speeds, respectively, compared with the traditional. Furthermore, the standard deviation of lateral error was reduced to 5.3 and 7.8 cm, respectively, in spray mode. The robustness and accuracy of the path tracking control of 4WSS unmanned agricultural aircraft were effectively improved in the complex farmland environment. Better anti-disturbance and strong adaptive ability were achieved to meet the needs of agricultural production.