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In meteorological observation projects, the automatic observation of frozen soil has not been realized. In order to solve this problem, this paper presents a method for measuring frozen soil by means of the changes of soil dielectric constant, based on the planar capacitive sensor. When the soil is frozen, the internal moisture will turn into ice, and for the dielectric constant of water is much larger than that of ice, the dielectric constant changes drastically after water freezing (phase transformation). Based on the characteristic of soil dielectric constant and ground temperature, the mathematical model to retrieve frozen soil is established, and a typical soil laboratory freezing test is carried out. The field comparison test results show that the frozen soil sensor can correctly distinguish the soil freezing state; the measured data and manual observation are of the same trend; the correlation coefficient is up over 099; the average measurement error is less than 3 cm. The sensor based on the dielectric characteristics of the frozen soil can accurately and continuously measure the soil freezing depth and various changes.