Abstract: Flory considered that the discrepancy between classical gelation theory and real reaction system was mainly ascribed to the intramolecular reaction. This paper used the basic principles of the probability and analysed the growing-disappearing conditions of the groups in the even generations against a certain group 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. Making use of the equations obtained from this paper to deal with the critical condition of gelation, we found that the theoritical values agree with the experimental ones.