A synthetical equation is proposed to characterize the essential features of the inverse “S” type curve on the oasis of summing-up simulation approach of “S” type curve.The two physical variables in the model obtained are discussed and the detailed method used to determine the parameters is given.The model is then presented to describe the crystallization of poly(caryleher ether ketone) (PEEK) and thermal decomposition of poly (amide-imide) (PAl) respectively.It is found that some thermal characteristic parameters can be well estimated from the model simulated in computer. A synthetical equation is proposed to characterize the essential features of the inverse “S ” type curve on the oasis of summing-up simulation approach of “S ” type curve. The two physical variables in the model obtained are discussed and the detailed method used to determine the parameters is given. The model is then presented to describe the crystallization of poly (caryleher ether ketone) (PEEK) and thermal decomposition of poly (amide-imide) (PAl) respectively. It is found that some thermal characteristic parameters can be well estimated from the model simulated in computer.