For the purpose of measuring micro strain of a stretched metal panel, a measurement system including a fiber sensor is designed, which works at 1550nm central wavelength. The fiber sensor is actually a small circle of multimode fiber, fixed vertically on surface of the metal panel. When the metal panel is stretched, the sensing circle produces strenuous bending loss effect, leading to violent fluctuation of light intensity of back Rayleigh scattering. The measurement system of axial micro strain is set up according to the principle of optical time domain reflectometer and the light intensity of back Rayleigh scattering is acquired so that the strain of metal material is calculated. It can also realize distributed and real time measurement. The experimental results show that sensitive radius range of the bending loss multimode fiber sensor is within 3~6 mm, calibration measurement range of micro strain is about 500~3000 με, measurement precision of micro strain is about 40 με.