A telescopic baby safety door barrier and an auxiliary mounting device are provided. The telescopic baby safety door barrier includes a door barrier main body, wherein the door barrier main body is provided with a mounting portion; a connecting hole is formed in the mounting portion; and the mounting portion is provided with a double faced adhesive tape. When the telescopic baby safety door barrier is mounted necessarily, only the mounting portion is adhered to a wall by the double faced adhesive tape, holes are then punched through the connecting hole, and screws are then mounted, thereby being convenient in mounting. The auxiliary mounting device includes a base provided with a sliding groove and a mounting jig slidably disposed in the sliding groove, and a first adjustment assembly for fixing the mounting jig is disposed between the mounting jig and the base.