There are theoretical and experimental results showing the existence possibility of the periodic solutions of the gas-condensate mixture filtration problem in porous media. In this paper consideration is given to the one-dimensional problem of the propagation of non-stationary waves in a porous medium with applying of boundary perturbances. This makes it possible to adequately calculate the quantitative parameters of the gas-condensate mixture flow process in the formation taking into account phase transition in the system. Boundary perturbance are given in the form of the Dirac delta function with the speed finiteness of non-equilibrium phase transitions and phase pressure fields coherence. A new approach explaining the formation of periodic solutions is proposed in this paper. It is shown that such behavior is typical for gas-condensate mixtures with a retrograde region at the phase diagram and zero values of phase permeability for determinate condensate saturation. Obtained results describe filtration unsteadiness of gas-condensate fluid in real reservoirs which has been observed during their operation