Abstract In this paper, the multi-photon FPE of driven optical systems are treated for bad cavity case. The semi-classical linear stability analysis are performed. The non-classical effects are discussed, and it is shown that in two-photon case the atom-atom correlation enhances the non-clasical effects. This is in opposite to the single-photon case, and different from the good cavity case. The two-photon spectrum of transmitted light and resonance fluorescense are obtained, where the Rabi frequency Ω∝y2 (y is the incident field).