In order to further understand the modal interference principle of fibe r optic sensors and improve the performance of the strain sensor ,a novel fiber sensor based on modal interferen ce of polarization maintaining fiber for strain measurement is proposed.A length of 30mm polarization maintaining fiber(PMF) is spliced with waist enlarge betwe en two conventional single-mode fibers(SMFs) to construct an all-fiber modal interferometer sen sor.The phase difference between the cladding and core regions of the fiber changes with the external strain increases,and then the interference spectrum changes.The optical spectrum analyzer (OSA) is us ed to defect transmission spectrum of a gain-flattened erbium-doped fiber with amplified spontaneous emission (ASE) through the sensor in the strain range from 0με to 3605με．The results show that the light intensity and the strain have highly linear relationship using the method of least squares linear fitting.The sensitivity can be 0.004dB/με,and the corresponding resolution is 2.5με．The presented sensor is with simple ma nufacturing,relatively low cost,solid structure and high resolution,so it is sui table for strain measurement in practice.