To address the difficulty of in situ and online monitoring of stem freezing information during the overwintering period of woody plants, an in situ and online sensor for the detection of stem volume ice content was developed based on the relationship between the change of water-ice ratio and stem impedance during stem freezing. The results of standing wave measurement frequency and circuit simulation showed that the circuit can effectively characterize the change of reactance component in stem impedance by using 100MHz signal excitation source; the analysis of impedance change during plant freezing showed that it was feasible to characterize the change of stem volume ice content based on the change of stem impedance. Probe electromagnetic spatial distribution simulation results showed that the sensor effective measurement area was 0~10cm; static and dynamic characteristics analysis showed that the sensor measurement range was 0~100%, resolution was 0.05%, measurement accuracy was ±1.76%, and it had good stability, the dynamic response time was 0.172s, and power consumption was 0.25W. The indoor and outdoor experiments showed that the sensor can effectively track the dynamic changes of stem volume ice content in the stem and the degree of freezing stress, reflect the physiological characteristics of the daily cycle of water-ice transformation during the overwintering period of plants, and it provided technical support for the monitoring of plant ice content and the cultivation of plantation forests under different latitudes.