Optical method for measurement of particulate concentration in flue basing on optical scintillation is an important non-intrusive technology. In order to research the influence of particles to optical intensity，the inten- sity degradation of probing beam is simulated basing on Lambert-Beer's law，when particle size distribution satisfies logarithm normal distribution and Gauss distribution，respectively. In the calculation，the relationship between ex- tinction coefficient and species and the middle level diameters are pre-calculated. The results show that，when the particle size distribution satisfies logarithm normal distribution，the intensity degradation of probing beam is more significantly. The optical intensity exhibites the same changes when the particle size decreases gradually.