The invention discloses a gas feeding system for a polysilicon reducing furnace automatically controlled by PLC. The gas feeding system comprises a PLC controller and an instrument gas source and at least one stop-type direction control valve in one-to-on correspondence to gas inlets of the reducing furnace. Each stop-type direction control valve is provided with a single electric control reversing valve; a gas outlet of the stop-type direction control valve is connected with one gas inlet of the reducing furnace; a gas inlet of the stop-type direction control valve is directly connected witha technical gas input apparatus; each gas inlet and one gas outlet of the single electric control reversing valve are connected with the instrument gas source, and another gas outlet of the single electric control reversing valve is connected with a control port of the stop-type direction control valve; and the PLC controller is connected with the single electric control reversing valve.