Automatic monitoring device, method and system for liquid level of oil well

The invention discloses an automatic monitoring device, method and system for the liquid level of an oil well. The automatic monitoring device comprises a plurality of wellhead collecting devices, a plurality of switching valves, a shunt and a main controller. A microphone is mounted inside each wellhead collecting device. A main pipeline and a plurality of output pipelines are arranged in the shunt. The switching valves control communicating and closing of the output pipelines in the shunt and the wellhead collecting devices. The main controller controls the switching valves to control communicating and closing of the output pipelines in the shunt and the wellhead collecting devices. The main controller collects electrical signals generated by the microphones and analyzes and calculates. the electrical signals to obtain the depth of the liquid level of the oil well. The wellhead collecting devices and the output pipelines of the shunt are connected through the switching valves in one to one correspondence. T