To correct measurement errors in spectral emissivity measurement based on FT-IR in the vacuum ambient, the emissivity calibration model has been constructed, including radiation thermal losses, one dimension heat transfer and radiation thermometry. The emissivity error has been quantitatively analyzed. The results show that the higher the sample temperature is, the lower is the value of the emissivity. This result is induced by thermal losses, material thermal conductivity and sample thickness. To reduce the error induced by these factors, the system should be calibrated. The calibrated spectral emissivity of high-purity graphite sample at 2000℃ agrees very well with the results reported by G. Neuer and G. Jaroma-Weiland.