Photodetectors are widely used in high technology field. Quantum noise is a main noise source of photodetector, and as an important factor in error analysis, it affects and decides detecting sensitivity. The difference and relation between noise and standard deviation are discussed. Based on the analysis of quantum noise sources, it is checked that the emitted photoelectrons have a Poisson distribution, but the multiplied photoelectrons do not obey Poisson statistics. Noise scale factor is introduced and used to describe and process the standard deviation of photoelectrons of photodetector.