We apply Wegner''s flow equation method to study the eigenvalue of two-level atom system acted by light field. Interactions not contained in the original Hamiltonian are generated by this unitary transformation. With an average field approximation, we can close the flow equations. In the flow parameter l→∞ limit with Hamiltonian H(l), the interaction terms vanish and noninteraction terms approach fixed values. In the end, we obtain an explicit expression of the spin atom autocorrelation function.