A more reasonable particle counter model suitable for airborne particle counter is constructed in the probability space.The amplitude spectrum line and the amplitude spectrum function are proposed for the airborne particle scattering light signal,and their properties are studied in probability space.Based on the irrelevance of the single distributing particle group and the probability theory of signal processing model,the essence of this signal transformation is summarized in a total probability equation.Experimental study has been performed with Y09 laser airborne particle counter.