Abstract: The signal transfer function(SiTF) is considered as the most basic and important parameter to evaluate the characteristics of the infrared thermal imaging system,and is the linear portion of the responsivity function. The information on gain, Linearity, dynamic range and saturability of UUT can be obtained from the test curves of the responsivity function. The Optic Metrology Station is currently engaged in a research on the evaluation of the synthetic characteristic of the infrared thermal imaging system．In this paper,the CEDIT IIA thermal imaging system is measured with the diblackbody and reflected target system. The equipment can ensure the uniformity and stability for the backgraound radiation，and also ensure it against the effect of the ambient temperature，so the accuracy and reliability of measurement are improved. Finally, according to the practical experience many years，the author lists the causes for variations in the measurement of SiTF.