To achieve high-accuracy temperature sensing, this paper designed a fiber temperature sensor with a composite structure based on the vernier effect and aluminum alloy substrate sensitization. The sensor is composed of a cascaded Fabry-Perot interferometer (FPI) formed by splicing a hollow-core fiber and two single-mode fibers, a fiber Bragg grating (FBG) in series connection with the cascaded FPI, and an aluminum alloy substrate. Its characteristics such as reflection spectra, transmission field, and thermodynamics are expounded by the three-beam interference theory, the beam propagation method, and finite element analysis. By controlling the geometric lengths of the hollow-core fiber and the single-mode fiber, we can flexibly adjust the vernier effect sensitization multiplier and temperature measurement resolution. The composite structure was designed to measure not only small temperature changes but also the absolute values of temperatures. The experimental results show that the temperature sensitivity of the cascaded FPI and FBG based on the vernier effect is 138.4 pm/℃ and 37.4 pm/℃, respectively, and the temperature repeatability and fast response of the sensor are good. The designed sensor can be widely applied to high-precision measurements.