Emissivity setting has a decisive impact on the accuracy of the measurement results of infrared thermal imager, so it must be calibrated accurately before temperature measurement. In this paper, the influence of emissivity setting on temperature measurement accuracy of infrared thermal imager was studied through thermocouple calibration test. Secondly, the influence factors of emissivity setting were analyzed by factorial experiment. Finally, the influence degree of each factor on the emissivity setting was studied by orthogonal experiment, and the empirical formula fitting and experimental verification were carried out. The results show that: for SiC_p/Al composites, the emissivity setting is very different before and after the measured temperature is 500 ℃. When the measured temperature exceeds 500 ℃, using the recommended value as the setting method will produce large errors. In the range of 0°-45°, the measurement angle has no influence on the emissivity setting. However, temperature, surface roughness and observation distance all affect the emissivity setting. The maximum error of the empirical formula of the emissivity setting obtained by polynomial fitting is 2.76%.