The paper proposes a mathematical model to estimate boost gas parameters at filling the feeding lines dead-end zones of the pneumohydraulic systems (PHS) of the liquid-propellant rocket engines (LRE). These calculations are required in preliminary assessment of the design characteristics of the PHS feed lines designed to ensure operation of the LRE pneumoautomatic systems. Engineering methodology currently existing to solve this problem provides for solution of a system of ordinary differential equations and does not allow tracking alteration in pressure and temperature along the length of the dead-end zones at filling the PHS feed lines. The proposed model takes into account inhomogeneity of the flow parameters along the length of the dead-end zones, as well as the nonadiabaticity of the flow, i.e. it assumes presence of the heat exchange between the walls of the feeding line and the boost gas. The presented model was used to construct dependences of gas parameters alterations in the feeding line. Results obtained were compared with calculations according to the evaluation method, which is traditionally used in the engineering practice to solve this class of problems.