Reliable interface pressure between cables and accessories is closely associated with the safe operation of a power cable system. Using an external optical fiber Bragg grating temperature-compensating curvature sensor, a new method for online measurement of the interface pressure between a cable and an accessory at high temperatures is proposed. On the basis of the sensing principle of a fiber Bragg grating, a mathematical pure bending model, and a model of a thick-walled cylinder, a fiber Bragg grating temperature-compensating curvature sensor is fabricated using Dow Corning Sylgard-184 silicone rubber. The interface pressures of 10 kV cable accessories with different expansion rates at different temperatures are measured. The results of the measurements show that the temperature-compensated deviation of wavelength measurements made by the fiber grating curvature sensor ranges from -2.59% to 5.50%. The interface pressure between the cable and the accessory gradually increases with an increase in temperature from 30 to 60 °C. The results are consistent with the results of measurements made using a traditional piezoelectric sensor. This method is expected to evaluate the long-term reliability of the interface pressure of cable accessories in actual complex-temperature environments.