A fiber optic sensing system that can be used to measure strainis designed and fabricated. The system demodulates the transmitted signals of two Mach-Zehnder Interferometersconnected in parallel based on the vernier effect, which greatly improves the sensitivity of strain measurement. The single Mach-Zehnder interferometer is based on a single-mode-no-core-few-mode-single-mode fiber structure. The experimental results show that the strain sensitivity of the single sensing interferometer is -1.90 pm/με in the strain range of 0~166.667με, the strain sensitivity of the envelope of the superimposed spectrum is 12.06 pm/με in the strain range of 0~166.667 με. The sensitivity amplification factor can be flexibly adjusted by adjusting the length of the sensing arms of the two interferometers to improve the sensing performance.The sensor has the advantages of high sensitivity and simple structure, which can be widely used in the field of precision measurement.