The invention relates to technical field of farm machinery and discloses an automatic guiding device for the farm machinery. The automatic guiding device comprises an electric steering wheel assembly, a vehicle-mounted control computer, a GPS receiver and an automatic motor steering controller. The electric steering wheel assembly is fixedly connected to a steering column of the farm machinery and is connected with the automatic motor steering controller. The automatic motor steering controller is connected with the vehicle-mounted control computer and the GPS receiver in sequence. The electric steering wheel assembly comprises a steering wheel, a driving motor, a sleeve and an optical-electricity encoder. The two ends of a rotor of the driving motor extend out of a stator. The sleeve is sleeved with the rotor. The lower end of the steering wheel and the upper end of the sleeve are fixedly connected with the upper end of the rotor through fastening parts. The optical-electricity encoder is fixedly connected with the lower end of the rotor through fastening parts. The automatic guiding device for the farm machinery is simple in structure, easy and convenient to install, low in energy consumption, friendly to environment, low in cost and high in guiding accuracy.