A method of improving the caculating speed of particle size distribution by reducing the number of photosensitive rings is proposed. Through analysis of light energy distribution diffracted by the particle and its derivation, a new formula of size class is introduced and the minimun of photosensitive rings is obtained. Theoretical analysis and numerical simulation demonstrate that the accuracy and measuring range of particle size with 6 rings is almost the same as those with 15 rings. The method is verified through practical measurements for 8 standard latices. It is very suitable for caculating particle size distribution with model dependent.