The atomic population inversion in the system of a two-mode SU(l,l)coherent field interacting with a two-level atom is investigated by solving Milburn equation. The effects on the atomic population inversion induced by the intrinsic decoherence,the two-mode photon number and the mean photon number are analyzed. The results show that the collapse and revival phenomena of the atomic population inversion will appear when the two-mode photon number is not zero and will disappear gradually with the decrease of the mean photon number. The collapse and revival phenomena of the atomic population inversion will also appear with the intrinsic decoherence,but the amplitude of the Rabi oscillation will be reduced to zero with the evolution of the time.