The Fourier-Transform infrared (FTIR) spectrometer is a commonly used instrument in infrared radiation measurements. An infrared spectral emissivity measurement facility using a deuterated tri-glycine sulfate (DTGS) detector has been set up at the National Institute of Metrology, China (NIM). The linearity of the spectral responsivity is one of the main uncertainty contributions in spectral-emissivity measurement, performed over a wide spectral radiance range (0–30,000 W/(m2·sr·μm)). Using the flux superposition method, a double-aperture method, where the apertures are positioned precisely by a cage structure, has been used to measure the linearity of the FTIR measurement system, a sub-system of the infrared spectral emissivity measurement facility, developed at NIM. The drift characteristics of the blackbody radiation sources and the FTIR measurement system during a typical linearity measurement have been investigated. Drifts of the blackbody sources are reduced by integration of the spectrum. The quasi-linear drifts of the FTIR measurement system are corrected by inverting the measurement sequence of the apertures. Moreover, corrections for the effects of aperture imperfections on the linearity measurement have been quantified experimentally. Measurements are reported for blackbody radiation sources in the temperature range (473–1273 K), with detection at the wavelengths 3.9 μm and 10.6 μm, respectively.