The sag of high-voltage transmission lines is an important indicator for the lines design and operation. Moreover, it is related to the safety of the lines operation and is controlled within the range specified by the design. Transmission lines sag measurement is a key research topic for domestic and international power departments and research organizations. In this paper, a capillary fiber-based high-voltage transmission lines sag sensor is designed. The sensor has advantages of a large mode area, strong anti-electromagnetic field interference ability, and real-time online monitoring. The experimental results show that when the sag radius of the sensor is less than 15 cm, the monitoring sensitivity is 2.552 dB/cm, and the linear correlation coefficient is 0.9635. The higher the sensitivity of the capillary fiber optic sensor, the smaller the sag curvature radius of the sensor and the greater the bending degree. The experimental phenomenon is consistent with the theoretical analysis results, indicating its practical applicability.