A novel hydrogen sulfide gas sensor based on titanium dioxide membrane-coated coreless fiber was presented. The sensor was fabricated by two different length no-core fibers (NCFs) which were spliced both ends with single-mode fibers (SMFs) and then constructed an interferometer with the structure of SMF-NCF-SMF. Different modes can be excited in the coreless fiber when the light traveled from SMF to NCF, an interferometer based on multimode interference was formed. The titanium dioxide film was coated on the outside surface of NCF which could adsorb hydrogen sulfide gas, then the relation between gas concentration and spectral shift was obtained, and thus the detection of hydrogen sulfide gas was performed. The experimental results show that a high sensitivity of 18.93 pm/ppm and a good linear relationship are achieved in the range of 0 to 60 ppm, and the interference spectra appear red shift with the increasing concentration of hydrogen sulfide. The rising time and falling time of the sensor are about 80 s and 110 s. The sensor has the advantages of simple structure, high sensitivity and easy manufacture, and can be used in the safety monitoring field of tracing hydrogen sulfide gas.