The actively heated fiber-optic (AHFO) technology has emerged as a frontier and hotspot in soil water content measurement, offering advantages such as easy installation, large-scale distributed measurement capability, and resistance to electromagnetic interference. However, current AHFO water content sensors fail to simultaneously achieve high precision, applicability for deep soil, and automated real-time monitoring, thereby limiting their development and application. Therefore, this study introduces a novel actively heated fiber Bragg grating (AH-FBG) cable. Laboratory tests were conducted to assess the heating uniformity of the AH-FBG cable and to establish the temperature characteristic value (Tt) - soil water content (q) calibration formula for water content measurement. Subsequently, AH-FBG cables were deployed for in situ soil water content monitoring in a test pit on the Loess Plateau. Through two-year monitoring data verified the accuracy of the AH-FBG cable and elucidated the spatiotemporal distribution of in situ loess water content. Laboratory results demonstrated superior heating uniformity of AHFBG cable, with a Tt standard deviation of approximately 0.3 degrees C. In the field, the AH-FBG cable exhibited excellent performance in soil water content measurement, achieving a high accuracy of 0.023 cm3/cm3. Further analysis revealed that the q fluctuation predominantly occurred within a 10 m depth from the soil surface, with an overall upward trend over the two-year monitoring period; the response of shallow q to precipitation was significant but exhibited increasing hysteresis with depth; frequent precipitation significantly enhanced water infiltration depth. This study provides technical guidance for highprecision, quasi-distributed, automated and real-time water content measurement of deep soil. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).