In order to get the frequency of periodic laser beat signal, the probability density function (PDF) of time interval between two successive photoelectrons is derived theoretically. The difference of the shape in the PDF curve between the beat signal and stable light signal is obvious, and the beat frequency can be derived from the difference. An experiment system is designed in which a multi-pixel photon counter (MPPC) is used to record the photoelectrons time of the beat signal with beat frequency of 6, 8, 10 MHz and 12 MHz to give a potent verification to the theory. The experimental result is even much better in the condition that the photons count rate is 1/5 of the beat frequency, and an estimation of the mixing efficiency can be obtain using the statistics method which is 0.65 in our experiment. In the limitation of the dark count noise of the MPPC, the minimum beat frequency of periodic signal is 0.2333 MHz detected using this method.