This article provides a method for calculating the thermodynamic parameters of the working fluid in a cylinder with cryogenic filling. The method for calculating takes into account the influence of the intensity of heat inflows to the outer wall of the cylinder, the change in the temperature of the outer wall, the thermal conductivity of the insulation of the inner thermos tank, the current values of the parameters of the liquid phase and saturated vapors above the liquid in the thermos, as well as gas in the gas cavity over time. Based on the proposed method, a program was developed for calculating parameters in a cylinder. To verify the method for calculating the state parameters in a cylinder with cryogenic filling, experimental studies were conducted, the results of which proved the applicability of the calculation method for predicting the parameters of the working fluid and the characteristics of the cylinder as a whole. Based on numerical simulation, the influence of the design parameters of a cylinder with cryogenic filling (thermos volume, wall thickness, thermos insulation thickness) and external heat inflows on parametric and time characteristics is determined.