This paper presents a new method for measuring target reflectivity. Different from the traditional direct measurement method, it is an indirect target reflectance measurement method based on the output voltage of the radiometer. The principle is that the target emissivity is first measured by a simple and convenient method, which use a radiometer to measure the blackbody, metal and target radiation voltage of the same size at the same temperature. Then the mathematical relation between reflectivity and emissivity is used to calculate the reflectivity of target. Experiments are designed to verify this method, a metal plate coat with stealthy nano-materials is selected as the measured target. The experimental results were compared with the standard arch method, indicated that maximum error is less than 5%. This is to say that the cost of the measurement scheme is reduced and the applicability is increased under the condition of ensuring the measurement accuracy.