The article evaluates the fatigue loading resource of the low-speed stage cylinder of a long-stroke piston unit when loaded by the internal pressure of the working medium in the injection zone. For low-speed compressor stages, an urgent task is to study the behavior of structures during long-term operation, since this machine has not been sufficiently studied and the purpose of its creation is an increased resource compared to existing multi-stage analogues. The results of the analysis of the operation of the compressor unit, obtained in the ANSYS Workbench program, make it possible to evaluate the influence of the properties of the material and design of the cylinder, as well as the characteristics of its loading on fatigue strength. The conducted research allowed us to create a general approach to the design of piston stages of units under the conditions of operational loads, which makes it possible to create piston stages with a given service life.