This study proposes a sheet strain sensor comprising two glass-fiber flakes, a silica fiber Bragg grating, and two glass-fiber tubes that is packaged by a high temperature-resistant adhesive. The strain sensor contains no metal material, facilitating resistance to strong power and electromagnetic interference. Three sensor prototypes are manufactured and arranged on a constant strength cantilever beam for comprehensive performance test. Experimental results show that all three sensors possess good measuring repeatability (repeatability error <3.43%), fitting linear correlation coefficients (>0.999), and consistency for measuring sensitivities. The sensors also demonstrate good temperature compensation ability (within 22 pm) and creep resistance. These test results indicate that the proposed sensor has a superior measurement performance and can meet the requirements of long-term strain monitoring for electric power facilities in the wild.