A composite structural strain sensor is proposed, which incorporates a high-sensitivity fiber Fabry-Pérot interferometer (FPI) into the fiber Sagnac interferometer (FSI). The sensitivity of the FPI strain sensor is greatly improved by the vernier effect, which is caused by the superposition of the FPI and calibrated FSI spectra. The results of theoretical calculation show that, the sensitivity of the composite structure sensor can be regulated by the free spectral range difference between two sets of interference spectra. Further, the experiment results show that, the sensitivity of the composite structure sensor is increased 19.7 times compared with that of a single FPI strain sensor, reaching 65.1 pm·με^-1. The composite structure sensor plays an important role in scenarios requiring high sensitivity measurement and accurate measurement.