The theory of temperature measurement by infrared imagers is introduced and the factors that influence the accuracy of infrared temperature measurement are analyzed. The error curves of temperature measurement at different surface emissivities are given. Using theoretical analysis, it is shown that the higher the surface emissivity of an object is, the more accurate the measuring accuracy of imager is. By changing the setting of surface emissivity, the corresponding total radiance luminance to surface emissivity is calculated. Analysing the results, the conclusion indicates that TP8 long wave thermal infrared imager can measure the temperature accurately when the surface emissivity is above 0.5. Finally, three materials whose surface emissivity is 0.96, 0.93 or 0.3 are measured, respectively, and the result shows that a higher surface emissivity leads to a better temperature measuring accuracy.