Abstract: In order to improve the efficiency of on-the-spot test and accuracy of the data, the authors proposed a new method to set up virtual noise sources based on the noise decay principle. Through this method, we can predict and calculate the noise level around the known points on the basis of the limited real measurement data, then draw the isoline map of propagation of the noise. The process is simple and convenient and the result very clear. Through a comparison of the prediction noise levels calculated by this method with actual values, the authors verify the precision and feasibility of this method. It reflects the situation and change of regional noise pollution in the noise pollution area objectively, and offers a new method for the on-the-spot ambient noise test of many noise sources.