The dynamical properties and spectrum characteristics of a Λ-type atom with an external field embedded in three-dimensional photonic crystals are investigated . With the help of numerical calculation and theoretical analysis, we find that the populations of the upper and lower levels display quasi-oscillation, and the frequency and amplitude of the oscillation is greatly affected by the intensity of the external-driving field Ω, the detuning δ　and the background decay of the upper level γ.　Ｓpontaneous emission of the atom is apparently influenced by the relative position of the upper level from the forbidden gap. Furthermore, the properties of the emission field around the atom are analyzed by means of s pontaneous emission spectrum.