A gas pressure sensor based on an in-fiber interferometer with a closed microcavity is proposed. Its temperature and gas pressure characteristics are theoretically and experimentally studied by investigating the dip wavelength of its interference spectrum. Based on the theoretical analysis, Fabry–Perot (F-P) and Mach–Zehnder interferences occur in the sensing process of this sensor. From our experiments, the wavelength sensitivity of this gas sensor is 83.6 pm/KPa at gas pressures from 101 to 120 KPa. And the hybrid interferometer has higher sensitivity than that of the single interferometer. This in-fiber sensor with a closed cavity has advantages of high reusability and simple structure, and it can not only reduce the optical power loss in transmission but also eliminate the influence of the gas refractive index. It can be used stably in a low-pressure monitoring system for flammable and explosive gas.