A Monte Carlo sampling algorithm fur the simulation of molecular structural evolution in a random crosslinking process of polymer-polyfunctional monomer (PFM) systems is proposed. Combining a simple gel content experiment with this algorithm, various important property changes in a tandem crosslinking reaction, such as molecular weight distribution, location of gel points, crosslinking density within sol and gel fractions, PFM molecular density within sol and gel fractions, etc., can be fully explored. A virtual polymer-PFM system with a very narrow initial molecular weight distribution and a commercial low density polyethylene-triallyl cyanurate system have been used to demonstrate the application of this algorithm. The implications of the simulated results were also discussed.