We present a hydrogen sulfide (H₂S) gas sensor based on a no-core-multimode-no-core (NMN) fiber structure. The sensor is fabricated by using two no-core fibers (NCF) which are spliced at both the ends of a multimode fiber (MMF), and the NMN structure is constructed. Different high-order modes can be excited when light travels from a single-mode fiber (SMF) to NCF. When light enters MMF, the high-order modes and fundamental mode transmit in the cladding and core of MMF, respectively, resulting in phase difference and mode interference. Simultaneously, the transmission characteristics of NCF and MMF having different lengths are optimized. The outside surface of NCF is coated with a thin titanium dioxide film by the dip-coating method; thus, a rapid response to H₂S gas can be achieved when the film absorbs the gas. With an increase in the H₂S concentration, the interference spectra denote a red shift. A sensitivity of 7.36 pm/10 ^-6 and good linear relation are obtained in the H₂S volume fraction range of 0-3×10 ^-5. In addition, titanium dioxide exhibits good selectivity to H₂S, and the response and recovery time of the sensor are approximately 50 s and 65 s. The sensor has advantages of simple structure, high sensitivity, and easy manufacturability, and it can be applied in the field of safety monitoring of the low-concentration H₂S gas.