The utility model discloses an automatically height-adjustable on-line moisture sensor, relates to a moisture sensor, and solves the problem that the moisture test data error of an existing moisture sensor is large. The moisture sensor comprises a sensor body, a connecting element, a floating plate and two guide rods; the sensor body is a moisture sensor which is arranged in oil in an oil tank; the connecting element is a round pipe connecting element with external threads, the bottom end of the connecting element is connected with the sensor body through screw threads, and a power supply and a signal wire in the sensor body are guided out of the oil tank by virtue of the connecting element; the middle position of the floating plate is connected with the external threads of the connecting element through screw threads, the top of the connection part is fixed by a lock nut, and two sides of the floating plate are provided with a guide rod hole respectively; the two guide rods penetrate through the guide rod holes in the two sides of the floating plate respectively and then are fixed in the oil tank vertically. The height of the moisture sensor can be automatically adjusted along with the liquid level change of the oil tank, the relative position of the moisture sensor and the liquid level of oil in the oil tank is enabled to be consistent all the time; the monitoring efficiency and accuracy of test data can be improved when the moisture sensor is used for carrying out moisture on-line monitoring on lubricating oil.