To suppress the distortion of speckle noise produced by laser illumination on image information in highspeed photography, the statistic characteristics of speckle noise are analyzed. Based on the statistic characteristics, incoherent superposition principle of laser beams to reduce noise is analyzed, specially when the average intensity of speckles is not identical and meets with different distributions. The contrast of speckle over the number of superposition speckles is discussed. The influence of the number of superposition speckles in different distributions of the average intensity of speckles on the contrast of speckle is simulated. The results indicate that, the contrast of speckle increases with the increase of the difference of the average intensity of speckle in superposition, leading to a worse suppression effect. And the theoretical results agree with the experimental results.