It is difficult to accurately measure the large-scale equipment surface emissivity，because it is difficult to control the temperature of the equipment.A method of emissivity measurement is proposed based on analyzing the main source of emissivity measurement errors.The radiation that reaches the detector is analyzed and derived,environmental IR radiation measurements are achieved by using two apparatus with known emissivity,thereby the environmental radiation on the emissivity measurements effects is eliminated. Experimental method is designed and emissivity measurements experiment is made，analysis results show that,compared with the method of no considering the environmental radiation,there error is reduced by 20.37%.The method has feasibility and accuracy for engineering measurement of surface emissivity.