Based on our previous work, in this paper, we discuss further the mechanism of maltiphoton dissociation of polyatomic molecules in an intense infrared laser field, and obtain non-linear equations which describe the coupling between molecular vibrational modes. Taking SF6 molecule as an example, we calculate numerically the energy transfer time between modes, the Fermi resonance interaction, and the degree of mode excitation by laser field. Finally, the dissociation rate of polyatomic molecule is discussed based on the uni-molecular reaction model, the theortical results are essentially in agreement with experiments.