The article proposes a method of parametric synthesis of the device of flattening the plant mass of the mower-flatter of the trailed KPP-4,2. A method of modernization of the parameters of the flattening mechanism has been developed, which can be used in both trailed and self-propelled mowers. It is established that the stability of plant mass flattening through the streets of the planting apparatus directly influences the energy costs of deformation of plant mass and indirectly the magnitude and rate of loss of moisture and nutrients in the flattened mass, which is largely determined by the structure and parameters of the mechanism of flattening (MP). The analytical solution to the design problem of the flattening device KPP-4,2 has an iterative implementation of the analysis procedures based on the formed FMM, as well as the procedures of comparison and selection of options in the procedure of parametric synthesis of MP. It is to choose a combination of internal parameters of the flattening device, according to the selected structure of the MP, which, while satisfying the functional constraints, simultaneously stabilizes the pressure from the upper to the lower roller. The analytical dependences of the operation of the trailed mower-conditioner KPP-4.2 on the height, density of grass stand, culture, agrotechnical requirements, field topography and physical and chemical composition of the soil have been established. Depending on the listed factors, the initial data and coefficients used in modeling change. As a result, we obtain the corresponding dependencies, show how the productivity and useful work of the mower changes from changes in the environmental parameters of the mowed mass and operating modes. Directly from the data obtained, it is possible to recommend the operating modes of the mower to obtain maximum performance on a particular background of work. As a result, we received optimization data to improve the operation of the trailed mower-conditioner KPP-4,2 taking into account all variables and coefficients.