The invention discloses an automatic gear shifting device of a vehicle. The automatic gear shifting device comprises an air pocket, a gear shifting air cylinder, a gear operating valve, an electromagnetic reversing air valve, a water pipe assembly, a first water level switch, a second water level switch and a main controller; a terminal K1 and a terminal K2 of the electromagnetic reversing air valve are electrically connected with the main controller; the water pipe assembly comprises a horizontal pipe, a first longitudinal pipe, a second longitudinal pipe, a first closing plate and a second closing plate; the first water level switch and the second water level switch are electrically connected with the main controller; when both the first water level switch and the second water level switch are disconnected, the main controller controls the terminal K1 of the electromagnetic reversing air valve to be electrified; when the first water level switch is connected and the second water level switch is disconnected, the main controller controls the terminal K1 and the terminal K2 of the electromagnetic reversing air valve to be not electrified; and when both the first water level switch and the second water level switch are connected, the main controller controls the terminal K2 of the electromagnetic reversing air valve to be electrified. The automatic gear shifting device is simple in structure and low in manufacturing cost, and does not require a driver to control manually, and the operation is very convenient.