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A multimode fiber-loop ring-down spectroscopy combined with a fiber mode converter for strain measurement is presented. The bending loss theory in multimode fibers is used for the explanation of strain induced ring down cavity loss mechanism, which agrees well with the experimental results. With this technique, strain measurement is achieved in a time domain by measuring the ring-down time. The strain sensing system is demonstrated with good linear response, good stability and high resolution as small as 0.33 muepsiv over a rang of 800 muepsiv.