We propose a novel strain sensing based on fiber-optic Mach-Zehnder Interferometer. A vortex beam is generated by a vortex fiber in the reference arm, and interferes with the fundamental Gaussian beam in the sensing arm to generate an interference spiral pattern. A camera is used to record the sequential frame of the spirals, and digital image processing technique is employed to recognize the rotation angle of the spirals induced by the strain under measurement. By this method, a strain resolution over 10-9 can be reached easily with a small measurement deviation. Besides, we compare the sensing characteristics for the cases of the optical vortex carrying different topological charges, and prove that the optical vortex with charge one fits our sensing application best.