A highly sensitive fiber optic Mach-Zehnder strain sensor based on vernier effect was proposed and prepared. The sensor consisted of two Mach-Zehnder interferometers with similar free spectral ranges connected in parallel. The single Mach-Zehnder interferometer was made by splicing a section of no-core fiber and a section of few-mode fiber between two single-mode fibers. Two Mach-Zehnder interferometers with similar but unequal free spectral ranges were superimposed in parallel to produce the vernier effect to achieve the effect of increased sensitivity. The experimental results show that the strain sensitivity of the single interferometer is -1.90pm/με in the strain range of 0~166.667με, and the strain sensitivity of the envelope of the superimposed spectrum of two interferometers in parallel is 12.06pm/με in the strain range of 0~166.667με. This sensing system adopts parallel connection to realize the vernier effect, which has the advantages of simple fabrication, high flexibility and high sensitivity, and can be widely used in the field of precision measurement.