Abstract: Flory considered that the discrepancy between classical gelation theory and real reaction system was mainly attributed to the intramolecular reaction.On the basic principles of the probability we analysed the growing-disappearing of the groups in the even generations against a certain group statistically when three different reaction systems containing two dynamic parameters reached some degree of reaction.Furthermore,we obtained the intramolecular reaction equations in the form of analysis by means of introducing probability factor of cyclization bond.Using the equations obtained from this paper to deal with the critical condition of gelation,we found that the theoretical values agreed with the experimental ones.