From the standpoint of stochastic Poisson processes, the separation of granular materials by size on multi-tiered sieve classifiers is studied. Taking into account the selected separation scheme, a system of stochastic differential equations is drawn up for the distribution function of particles along with the sieve of the classifier. The coefficients of the differential equations are determined depending on the probability of sifting particles into the sieve cells. On the basis of the constructed mathematical model and the posed optimization problem, a technological calculation of the sieve classifier is carried out to determine its design and operating parameters, depending on the shape, size, and other characteristics of the material being separated.