The relativistic formalism of Green's functions is discussed in QCD and QED, where the relativistic Green's functions are constructed using the Schwinger proper time formalism and analyzed using the Fock–Feynman–Schwinger method. As a result, a simple and exact method is found for the relativistic systems, where the interaction can be written in a time-independent form. In this case one can write the relativistic Green's function as a one-dimensional integral of the corresponding nonrelativistic Green's function. The explicit example for the problem of a charge in the constant magnetic field is discussed in detail, and the exact agreement with the Schwinger relativistic form is demonstrated. A similar analysis is performed in the relativistic Coulomb problem, supporting the accuracy of the proposed relativistic formalism.