The eigenfuctions and eigenvalues of a cascade three-level atcm irradiated by two light fields are derived. The formulae for evolution of the atom are obtained. The changes of the populations of atomic levels and of the polarisation of the atom with respect to atomic levels and of the polarization of the atom with respect to the intensities of the fields are studied. The influence of the phases of fields and the atomic wavefunction on the dynamics of the atom is discussed. When a relative phase is equal to π the interaction between the atom and the fields reachies a minimun. In certain circumstances the minimun may be zero, which indicates no coupling between them.