The utility model relates to an automatic braking device of a car oil brake, which comprises a braking oil groove and an oil press brake of the automatic braking device of an oil brake, wherein a hydraulic pump and a pressure sensor are arranged on an oil entering pipeline between the braking oil groove and the oil press brake, an electromagnetic valve is arranged on an oil return pipe between the braking oil groove and the oil press brake, and a hydraulic pump switch which controls the starting of the hydraulic pump and an electromagnetic valve switch which controls the starting of the electromagnetic valve are arranged under a seat of a drive. Because the hydraulic pump switch which controls the starting of the hydraulic pump and the electromagnetic valve switch which controls the starting of the electromagnetic valve are arranged under the seat of a drive, after the driver leaves the seat, the hydraulic pump switch is started up to make the hydraulic pump work, braking oil in the braking oil groove is pressed into the oil press brake, after certain pressure is reached, the pressure sensor closes the hydraulic pump, and a car is braked then. When the driver returns to the seat, the electromagnetic valve switch which controls the electromagnetic valve is started to open the electromagnetic valve, braking oil in the oil press brake returns to the braking oil groove for releasing brake, and the car can be started. The utility model avoids the generation of traffic accidents caused by the automatic slide of cars because a driver forgets to pull a hand brake.