The invention provides a grounding sensor and a method for automatic water vapor medium identification. The grounding sensor comprises front and rear chambers, and a gap is reserved between the front and rear chambers. The front and rear chambers are internally provided with emission and reception ultrasonic wafers respectively, the two kinds of wafers are positioned at two sides of the gap respectively, and the chambers are provided with a magnetic proximity switch and a detection circuit which is composed of an ultralow power consumption single-chip microcomputer, an ultrasonic detection module, and a grounding detection module. The method comprises steps of installing the sensor on a hoisting mechanism of a grounding device, detecting grounding by a distance between a magnetic part and the magnetic proximity switch, and detecting water entry using an attenuation difference between ultrasonic waves in air and water. The invention adopts a testing method of combining ultrasonic detection and the magnetic switch, and puts misoperation caused by the grounding process on an investigation ship deck to an end; the ultralow power consumption single-chip microcomputer is adopted to greatly improve the reliability and the stability of output signals of the sensor; and states of four kinds of signals output by the sensor can be applied to determination of the working operation of marine measurement instruments.