The signal-to-noise ratio formulas of a multi-pixel photon counter (MPPC) are obtained based on its photoelectric characteristic and experimental results, and by revising the traditional signal-to-noise ratio (RSN) formula of linear mode avalanche photodiode (APD). The method is used for random photon detection mode and short-pulse photon detection mode, respectively. The numerical calculation results using the obtained RSN formula are consistent with the experimental results. The numerical results show that the minimum detectable photon flux is of the order of 103 count·s-1, the minimum detectable power is about 10-15 W. Numerical analysis for the RSN formula of the short pulse photon detection mode is done. The results show that the RSN of the MPPC can be obviously improved by adjusting the photoelectron-number equivalent threshold of the MPPC. This method of the MPPC has its own advantage compared with the traditional photon detector. It has wide application prospect in the field of pulsed low-level light detection such as pulsed laser ranging and high-energy particle physics.