A standby battery box for an electric cylinder is electrically connected to a control box for driving the electric cylinder and includes a charge-discharge device and a rechargeable battery. The charge-discharge device includes a protection unit, a power conversion unit, a voltage detection unit, a control unit, a discharge unit, a display unit, and a switch unit. The rechargeable battery is electrically connected to the charge-discharge device. When a startup switch of the switch unit is pressed, the charge-discharge device delivers the electricity of the rechargeable battery into the control box. When a shutoff switch of the switch unit is pressed, the charge-discharge device does not supply power, thereby protecting the standby battery box from being exhausted.