The analysis of the technological process of operation of the feed unit of the fodder harvester is carried out based on theoretical methods. The methodology and calculation of the functional mathematical model of the pre-pressing mechanism of the feed unit are given, taking into account the dependence of the thickness of plant mass layer passing between different pairs of rollers upon physical - mechanical parameters of harvested crops The fundamentals of the algorithm developed by the author are used at RKUP « GSKB for Grain and Fodder Harvesting Machinery» for the computer-aided design of roller feed units.