The presence of noise has a great impact on the accurate measurement of electromagnetic radiation. The influence of noise cannot be ignored especially for the measurement of electromagnetic radiation of some weak signals. At present, the way to correct the electromagnetic radiation measurement results is to reasonably set the parameters of the spectrum analyzer, which is not universal because there exist differences in parameters setting for different spectrum analyzers. In this paper, by studying the characteristics of the base noise of the spectrum analyzer and the noise in the measurement environment, a suitable noise model is built and its rationality is proved through experiments. This method can provide a new way for the assessment and measurement of electromagnetic field exposure.