The statistic properties of photons in a system of two-mode squeezed vacuum field interacting with a two-level atom are studied without the rotating-wave approximation by means of quantum theory, and the influence of the virtual-photon process on the statistic properties of photons is discussed emphatically. The results obtained using a numerical method indicate that the influence of the virtual-photon process on the statistic properties of photons qives rise to quantum chaos, that is related to the initial state parameters of the system.