The high-frequency ultrasonic pulse generated by the sensor (probe) of the liquid level controller can pass through the container wall. This pulse will propagate in the container wall and the liquid, and will be reflected back. Through the detection and calculation of this reflection characteristic, it can be judged whether there is liquid in the container at the monitoring point. At the same time, the liquid level controller can output a relay signal to the subsequent electrical appliances or other equipment, so as to realize the monitoring or control of the liquid level.