The invention discloses a device for automatic unloading of a grain harvester and a control method. The device comprises a controller, a control panel, a granary fullness sensor, an unloading cylinder, a high limit sensor, a transmission mechanism, a rotary driving mechanism and a lifting driving mechanism, wherein the transmission mechanism is used for driving a stirring cage in the cylinder, therotary driving mechanism is mounted on the side edge of the vertical unloading cylinder and used for driving the vertical unloading cylinder to rotate, and the lifting driving mechanism is used for driving the vertical unloading cylinder to rise or fall. A granary sensor replaces manual visual observation to judge the grain fullness state of a granary, a driver can concentrate on driving operation, labor intensity and operation complexity are reduced, and automatic unloading and cylinder rotating and automatic returning functions can be achieved.