In this paper, a molecular weight model in the vinyl chloride (VC)-divinyl monomer suspension copolymerization was derived from the mechanism of VC two-phase polymerization, with pseudokinetic constant method and the theory of the moments of chain length distribution. Furthermore, the behavior of average polymerization degree was simulated with the model at varied experimental parameters. The simulation phenomena were discussed in detail. It is concluded that our model can be useful to predict the behavior of DP, and to look inside the crosslinking mechanism in the VC-divinyl monomer copolymerization.