The theory of coupling model was used to analyze the transmission properties of fiber grating,and matrix method was introduced to simulate the value of the reflection spectrum of uniformity fiber grating.The origin of noise in optical detection was analyzed,and the noise level of laser used in lab was evaluated and validated.By putting different power noise on lamp-house,the influence caused by lamp-house noise on detection of peak value was researched according to the refection spectrum of fiber grating.It was experimented that the demodulation precision of fiber grating was infected greatly by power noise of laser,and the higher the precision of wavelength detected was,the smaller the level of noise should be.When the demodulation precision reached the level of picometer,noise of laser should achieve the high-point of quantum noise.This paper provides theory foundation for the application and research on fiber grating technology used in high precision detecting of weapon equipments.