We theoretically study the photon counting statistics characterizations of the real single molecular photon source with a Poisson statistics background and that of an ideal double molecular system based on the standard HBT detection. Mandel parameter Q of each source with the same mean photons n—was analyzed. We have obtained an efficient method to identify single molecule by the two-photon statistics probability P(2), which shows the relation P(2)2. Additionally, we studied the effect of single-to-background ratio of the single molecular photon source on the criterion.