Based on the evanescent wave coupling technique,a Mach-Zehnder interferometer (MZI) with the size of 800 μm×200 μm was constituted by using two micro-nanofibers.The sensing arm was coated with palladium which is sensitive to the hydrogen,with a length of 200 μm.By optimizing the coupling lengths of the interferometers via precise micro-manipulating,the extinction ratio of an interferential wave can be up to 15 dB,and thus a large-dynamic optical sensing range is achieved.