The author work out the differential equation of the density matrix of the two level atom system in accordance with the Raman process of the two- level atom interacting the two- mode fields, then make research on its decoherence properties. The result reflects that the decoherence of the quit has something to do with its original state , the frequency of the two - mode fields and the average quantity of photon in the two - mode fields . Decoherence phenomenon is especially o...