The laser disturbing effect under the different F number of optical system has been investigated in the experiment. We measure the noise grey value and its distribution probability of CCD output image under the dark background and no laser disturbing. According to comparing laser spot image and grey scale distribution under the different F numbers, it is found that the pixel number of different grey scales linearly increase with the gradually decrease of F number, not exponentially or logarithmically. At last, the diffraction pattern of pentagonal stop is effectively simulated based on the theory of Fraunhofer diffraction. The simulated results show that laser spot image is just the diffraction pattern of stop at the large F number. The research results can provide technical support and theoretical reference for evaluating laser disturbing effect of electro-optical imaging system under the different F numbers.