In this paper, a highly sensitive fiber optic strain sensor based on the Fabry-Perot interferometer vernier effect is proposed and prepared. The sensor consists of two parallel Fabry-Perot interferometers, where the sensing interferometer consists of a single-mode fiber and a short segment of capillary quartz tube fused together, and the reference interferometer is aligned by a single-mode fiber placed in a fiber fusion machine. When the two interferometers have similar free spectral ranges, the strain sensitivity of the sensing interferometer is further amplified by using the vernier effect. Based on this, the parallel structure can be selected with different magnifications to obtain different sensitivities by simply adjusting the fusion splicer to change the reference interferometer cavity length. The experimental results showed that the strain sensitivity of the parallel structures in the strain range of 0~250 με with the assistance of the fusion splicer reached 22.16 pm/με and 32.88 pm/με, which were 3.84 and 5.70 times higher than the sensitivity of the single structure (5.76 pm/με), respectively.The sensor has the advantages of simple fabrication, high sensitivity, good repeatability, easy operation, and low cost, which provides a new idea for strain detection.