To minimize the uncertainties related to the FTIR spectrometer, three improved methods were proposed to measure the spectral emissivity. For method 1 (M1) and method 2 (M2), an invented device was used to change the environment radiation which makes the ill conditioned equation solvable. Method 3 (M3) is based on the gray body assumption, and an iterative procedure was applied to derive the temperature of samples, then the spectral emissivity can be retrieved based on this temperature and the FTIR measurements. In order to validate these methods, the emissivity derived from the measured temperature was used as a reference. The results indicate that for the samples with high emissivity, such as quartz plates and cardboard, three improved methods have comparable accuracy; while for the samples with low emissivity, such as rusty iron plate and aluminum plate, the M3 is more reliable than the other two methods. M1 and M2 need to go through more procedures than M3, and this may lead to some deviation from the reality. Overall, three improved methods greatly improved the spectral emissivity measurements with FTIR spectrometer.