The priming process of feed lines of cryogenic propellant liquid rocket engines was studied. The two-phase flow of cryogenic propellant priming process was treated homogeneously as pseudo-fluid with average properties. A feed pipe was divided into uniform finite elements. By applying the basic conservation laws to the elements filled with propellant or purge gas, and employing the equivalent flow-capacitance equation to deal with the two-phase element, the finite element state-variable model for the priming process of the cryogenic propellant feed pipe was developed. By treating vessel as two-phase element, we also obtained the equations of its priming process. Using our models, the processes of LH₂ and LOX priming a pipe were calculated respectively. The calculation results are given.