The invention relates to a hydraulically-driven steering device, in particular to an electrically-controlled hydraulically-driven steering device for a mobile platform of an agricultural robot, which comprises a supporting plate, a double-acting single-piston-rod cylinder, a connecting rod, a hydraulic steering gear, a coupler, a stepping motor, an oil tank, a remote controller, a three-position four-way electromagnetic directional valve, a gerotor motor, a filter, a gasoline engine, a dual gear pump, a speed regulating valve and a one-way valve. The stepping motor is connected to a power input shaft of the steering gear through the coupler, the steering gear is a parallel four-rod steering gear, and the remote controller is used for controlling steering and turn angles of the stepping motor to change the expansion length of a piston rod, so that turn directions and the turn angles of wheels are controlled. The remote controller is used for adjusting the position of a valve spool of the three-position four-way electromagnetic directional valve to control steering of the hydraulic motor, so that advancing, retreating and stopping of a four-wheeled vehicle are realized. The speed regulating valve is added in front of the stepping motor so that stepless speed regulating can be realized, and the electrically-controlled hydraulically-driven steering device is simple in structure, convenient in operation and low in cost.