Molecular nano-junction is modeled by 'metal/single molecule/metal’ structure. With the generalized master equation method, the inelastic current induced by the sequential charge transmission through the junction is studied under the excitation of infrared fields in the case of weak lead-molecule coupling.The interaction between infrared field and molecule is described with the models of exponential coupling, square coupling, and linear coupling respectively. In the excitation of varieties of infrared fields, the inelastic current-voltage characteristics and the effects of intramolecular vibrational energy redistribution are discussed.