The invention discloses a PLC-based 110kV transformer substation DC power supply system backup device comprising a PLC programmable controller, a 24V DC switching power supply and auxiliary relays K1 and K2. The 24V DC switching power supply supplies working power to the PLC programmable controller. When main and backup DC chargers fail to work or storage battery set end voltage decreases due to the abnormal condition of a centralized monitor, the device can automatically conduct the DC power supply output on an test power supply screen in the station and a DC closing bus in the station, and an emergency backup charging power supply is provided and accident alarm signals are uploaded to a master station and a substation background machine so as to strive for precious time for taking emergency defect eliminating measures for operation and maintenance personnel.