This paper introduces a kind of frozen soil and dry soil layer automatic observation sensor,which mainly consists of the control unit, frequency acquisition unit and temperature acquisition unit. The sensor realized frozen soil and dry soil depth measuring, based on the capacitance type soil moisture measuring technology and semiconductor temperature measurement technology. According to the soil water content and soil temperature, the sensor can automatically judge the soil layer whether is frozen or dry, and realize continuous automatic observation. The sensor can provide objective and automatic observation data for agricultural production management, environmental protection, transportation construction and climate change research.