A gas sensor based on the Mach-Zehnder interferometer with copper-deposited tungsten disulfide coated thin-core fiber is proposed. The fiber-core mismatch-type gas sensor is formed by fusion of standard single-mode fiber and thin-core fiber. The relation between the gas concentration and the spectral shift is established by the adsorption of the copper-deposited tungsten disulfide film to the hydrogen sulfide gas. Therefore, the detection of the hydrogen sulfide gas with lower concentration can be achieved. The experimental results show that the detection sensitivity of the hydrogen sulfide gas is 29.3 pm and the sensor has a good linearity and selectivity within the gas volume fraction range of 0~6×10 -5. The sensor is especially suitable for online monitoring of hydrogen sulfide gas with low concentration for the advantages of simple structure, high sensitivity and easy to manufacture.