Along with the progress of fine-graining, the rule of particle size distribution in fine-grained tailings becomes increasingly significant as it has an important influence on tailings' macro mechanical properties and stability. In this paper, we analyzed 82 tailing samples with the sieving and the settlement methods, and set up a Weibull function model for fine-grained tailings. The experimental results show that the number of fine particle is far more than that of coarse particle, and this distribution character can be described and speculated by Weibull distribution. Weibull distribution could be regarded as a fractal model to a certain extent and it considers the change of crushing rate that tradition fractal model doesn't consider. In the process of crushing, the peak of probability density distribution shifts towards the direction of fine particles, and this explains how the distribution character of fine-grained tailings forms. It's hoped that the research findings provide references for granularity theory and ore grinding process.