The accuracy of infrared temperature measurement is difficult to reach a satisfactory level, due to the problem of accurate determination of emissivity in an inhomogeneous temperature field is not resolved. The proposal of an emissivity measurement method is very important. Therefore, the emissivity of non-metallic and metallic materials needs to be determined by a measurement method. This study aims to provide a widely theoretical and experimental method for measuring emissivity under the inhomogeneous temperature field. The influence of the object emissivity and the ambient temperature field on the measured temperature is analyzed by the experimental method and theoretical model. Meanwhile, compared with conventional methods, the emissivity results obtained by using new methods in this study can effectively avoid the influence of uneven ambient temperature field on temperature measurement accuracy. In addition, the change rule of emissivity and temperature of the forehead skin in the state of dust, sprinkling water and wind blowing are also studied. Our results reveal that surface temperature, ambient temperature, and surface state of the subject must be considered if tester want to guarantee the accuracy of the instrument's measurements.