The relevance of this study is conditioned upon the need to increase the amount of high-quality seed material of small-seeded crops, which cannot be achieved without an important process of separation. The separation is based on technical and technological principles of separation of seed material, depending on the differences in physical and mechanical properties of individual components of seed material. Cylindrical triers are used to separate seeds according to their length. The purpose of the study was to find the theoretical regularities of changes in the technological parameters of separation of the seed material of small-seeded crops from the structural and mode parameters of a cylindrical seed trier by numerical modelling. The solution of the set problems was implemented in the STAR-CCM+ programme based on the finite element method. Considering the physical and mechanical properties of the seed material, as a result of the study, a visualisation of the separation process was obtained depending on the research factors: the rotation frequency of the trier cylinder, the diameter of the cylinder, and the number of seeds and impurities in the seed mixture. According to the simulation results, the dependence of the relative content of impurities ε on factors under study was obtained in the form of second-order regression equations. Using the Mathematica programme, a compromise problem was solved, which lies in minimising the multiplicative function of the research criteria, and the rational constructive and technological parameters of the trier separator were found. The material of this paper is intended for scientists, graduate students, designers of agricultural machinery, students, and specialists in agricultural production.