In this paper we investigate a coupled system of an initially excited two-level atom coupled to a monomode cavity,and compute spontaneous emission spectrum and mean population density emitted by the atom and the cavity using the quantum theory that accounts for theoretical derviation and numerical calculation.The spectra emitted separately by the atom and the cavity in of strong coupling region are dramatically different from those in weak coupling region at resonance.The cavity spectrum line splitting in the strong coupling region,however,a hole exists in the atomic spectrum in the weak coupling region.We study systematically the emission spectra when the atom and the cavity are detuned,and the results show that if the cavity linewidth is much smaller than the atomic linewidth(good cavity regime),photons are emitted at the cavity frequency even if the atom and the cavity are strongly detuned,and provide theoretic foundation for the apparently puzzling feature observed in the recent experiment.In this paper we computate the atomic and the cavity mean population densities each as a function of time and their relation to the luminous intensity, and offer a new insight into civity induced transparency effect.