The principle of this kind of automatic ice thickness monitoring system is realized by detecting the vertical distance between the ice bottom and a fixed point in the water, which is measured by using a barrel moving from the fixed point to the ice bottom. A water pressure sensor equipped in the barrel can detect the location of the barrel and the barrel movement is controlled by a pump, which can change the barrel's gravity by sucking or draining water. The instrument can record the sensor signal and set time of the pump switch. In order to avoid the impact of barrel movement and ice freezing on it, only when measuring the location, the barrel floats to the ice bottom and the barrel locates in the fixed point in other time. This paper presents the specific theory of the system and the preliminary data analysis of on-the-spot experiment.