Abstract: Theoretical research on the record of every incident output of the singlemolecule photon source responded by two SPCM with standard HanburyBrownTwiss technique is presented. The photon statistic measurement of a single molecule photon source based on the HBT configuration with an unbalanced detection system is studied. The photon counting statistics character of the real single molecular photon source with a Poisson statistics background is theoretically investigated. It is shown that the unbalanced detectors, the unbalanced beamsplitter and the unbalanced linear propagation efficiencies will cause the single event Mandel parameter to be less than that of balance system. In addition, the correction expression to the Mandel parameter based on unbalanced detection is given.