The invention provides a device and a method for measuring spectral emissivity of an infrared sample material at high precision. The test requirements for the emissivity of various sample materials in different sizes can be met by adopting a spectrum comparison measuring method and designing an optical coupling system and a water cooling variable diaphragm, the selective amplification of a signal is realized by virtue of a method for laminating and calculating signals measured on a same wavelength point for multiple times, the noise and system stray light of an environment background can be effectively simulated, the measurement precision can be improved, and the high-precision test on the spectral emissivity of the sample material can be realized. By adopting the method, the high-precision measurement difficulty of the spectral emissivity of the infrared sample materials in different sizes can be solved. The method has advantages of high measurement precision, wide temperature range, applicability to different sample materials in different sizes and the like. By adopting the device and method, not only can the accuracy in testing the spectral emissivity of various infrared materials be satisfied, but also an guiding significance on metering test of an infrared target, the infrared simulator spectral emissivity and infrared target spectral radiance can be realized.