Based on the theory of non-linear vibration and conception of uni-molecular reaction, we suggest in this article a physical model to describe the phenomena of instantaneous dissociation of polyatomic molecules interacting with an intense ir laser field. It is shown that there exists close and internal relationship between the dissociation threshold usually observed and the "jumping" processes occured inherently in nonlinear vibration. Taking the SF6 molecule as an example, the dissociation threshold, frequency "red shift" and the rotational compansation etc. are analyzed and evaluated. Good quantitative agreement between theoretical results and experimental observations is found.