The conception of mass subset is proposed for aerosol which also has been extended to the equivalent mass subset corresponding to counting signal channel. According to statistics, the fractal measurement model of aerosol's equivalent volume for counting signal is established. Due to the aerosols' shape distribution of different mass subsets showing the statistical self-similar character, the calculating formula is given for aerosol equivalent mass of signal channels in the process of aerosol mass measurement. Based on this, the theoretical basis is founded for aerosol mass measurement by using counting method.