To address the need for the long-term and stable monitoring of regional soil moisture in challenging outdoor power supply situations, a soil moisture sensor based on an expansion-type fiber Bragg grating (FBG) was developed. The sensor’s sensing element was fabricated from a hydrophilic rubber material. Three different rubber probe head structures were tested and compared to ensure that the mesh-penetrating rubber probe head was selected for its superior water absorption and desorption characteristics, monotonicity, and sensitivity. The effects of temperature on the sensor’s measurement results were analyzed, and temperature compensation was implemented using a dedicated FBG with a thermal sensitivity of 0.35 pm/K. The dynamic response performance of the sensor was evaluated using a spectrometer, which showed response times of 5 h for the water absorption phase and 8 h for the desorption phase. The sensor’s effective output wavelength ranged from 1557.711 to 1557.642 nm. To validate its effectiveness, Beijing’s sandy loam soil was used as a test sample, and linear regression analysis was conducted between sensor output values and corresponding measurements. This analysis yielded a coefficient of determination (R2) of 0.9659, indicating a strong linear relationship. Utilizing this designed sensor, we established a soil moisture measurement system that enables long-term and stable monitoring in regional contexts even under challenging outdoor environments. Field tests confirmed that this sensor enables reliable, long-term monitoring with high immunity to external interference and independence from local power supplies.