The interactions among R35, T34 and S54 processes in a six-level system were studied. Near the cross point of the resonant curves of these processes in the multiphoton resonance diagram of the six-level system, such interactions should be very strong. We found theoretically that in the case of off-resonant-pumpnig, the interactions among these multiphoton prooesseg should also lead tosplitting of the laser spectral line, which is called off-resonant-pumping AC Stark effect. Because tJhe influenoes of saturation eifeot and electric dipole transition enhancement effect were not the same for different mnltiphoton processes^ oontributions of different nmltiphoton processes to the o￡f-resonant AC Stark effoot could be distinguished by the injection of FIR signal into the system