A two-dimensional photonic crystal optical attenuator with temperature control based on the Fabry-Pérot cavity is proposed. The structure utilizes the linear modulation of the refractive index of the photonic crystal induced by thermooptic effect of the doped silicon. The change in refractive index causes variations in the photonic crystal bandgap and optical path difference of the Fabry-Pérot cavity. This causes linear variation in reflectivity as a function of temperature within a specific temperature domain, which realizes the controllable attenuation of the incident light. A commercial electromagnetic simulation software, CST, is used to simulate the reflectance spectrum of the photonic crystal with the Fabry-Pérot cavity after silicon doping, showing that the spectrum curve shifts as a function of temperature. The variation pattern of reflectivity is simulated for this structure as a function of temperature at several specific frequencies. It is found that the curve has relatively good linearity at a frequency of 0.284 THz. The simulation is conducted for incidence angles of 1°, 3°, 5°, and 8°, respectively, whereas the linear fitting is performed for the temperature-reflectivity curve at 3°, through which the characterization formula is obtained. A temperature-controlled photonic crystal optical attenuator with a tuning amplitude of 0-7.68% is designed.