A novel fiber optic multimode strain sensor based on self-imaging effect is presented.The longitudinal strain can be measured by the shift of peak wavelength of transmission spectrum satisfying self-imaging condition.The impact of structure parameters of multimode fiber on the transmission spectrum is analyzed.Experimental results of strain ranging from 0 to 4 400 με show a good linear relationship between the strain and the peak wavelength,and the strain sensitivity is 1.29 pm/με.The sensor is easy to fabricate,and the precision requirement of the length of multimode fiber is not too rigorous.Due to the pass-band filter response of multimode interference(MMI) devices,the multiplexing can be easily implemented by the parallel connection of no-core multimode fiber sensors with different lengths.