The invention discloses a door-locking power-off device, which comprises a normally closed electromagnetic relay, wherein a normally closed contact of the normally closed electromagnetic relay is connected in series to a circuit of a service power supply; a control circuit consists of a relay power supply and a control switch which are connected in series with an electromagnet of the normally closed electromagnetic relay; the control switch is a normally open contact switch; and a control point of the normally open contact switch is arranged in a lock catch box of a service door lock to correspond to the end part of a lock lever. The device adopts the control circuit consisting of the conventional 3V or 1.5V power supply and the normally closed relay; when a door is locked, the contact ofthe relay is disconnected to control the cut-off of the service power supply flexibly, save energy, and prevent occurrence of a fire disaster and other safety accidents; meanwhile, the device can eliminate personal fatalness, has a simple structure, flexible reaction and low cost.