We study the dynamic properties and algebraic invariants of the model of phase modulation of quantum light by microwave radiation. The quasi-energy levels of the system are described by the eigenvalues of the effective  Hamiltonian, which generators obbey to su(2) algebra and have a nontrivial internal structure. The dynamics of states is studied in Fock space. Invariant Hamiltonian spaces are associated with irreducible representations of the su(2) algebra, within which the Hamiltonian matrix is partitioned into finite blocks. Within this model we investigate the generating process of two-mode entangled states using the single-tone Fock state at the input of phase modulator.