Different from the traditional discrete spectrum temperature measurement, the essence of thermal imager temperature measurement is a kind of continuous spectrum temperature measurement based on the spectral response of detector in a certain band. In addition to the emissivity of the target will affect the relationship between the temperature measured by the thermal imager and the real temperature, the reflected environmental radiation is also an important factor affecting the temperature measurement results of the thermal imager. The current research on the temperature measurement accuracy of thermal imager, in the processing of the environmental radiation reflected by the target, mostly regard the incident environmental radiation as uniform or the target as a mirror reflector. Although this can greatly simplify the model, it is far from the actual situation. Based on the principle of radiometry and the spectral response characteristics of thermal imager detector, the temperature measurement model of the thermal imager for the diffuse target was established. The results show that the influence of environmental radiation on the temperature measurement of thermal imager is not only related to the reflectivity of the target and the temperature of the environmental radiation source, but also related to the projection solid angle of the environmental radiation source on the target. According to the model, the influencing factors of the temperature measurement of the thermal imager were analyzed. Then the influence of several typical environmental radiation conditions on the temperature measurement of the thermal imager was calculated and analyzed, and the experimental verification was carried out. On the one hand, the results can provide a reference for the design of temperature measurement of thermal imager, on the other hand, it can provide a theoretical basis for the error estimation between temperature measurement of thermal imager and real temperature under complex environmental conditions.