There exist intra- and intermolecular reactions in a controlled cross-linking system, and whether cross-linking process and molecular weight of product can be controlled depends on the two reactions. Soluble cross-linked macromolecule is synthesized by changing material ratio resulting in that the number of intermolecular reaction is controlled to a certain extent and molecular weight can't increase illimitably. Though material ratio is a common chemical reaction condition, it has been proved that it can influence and control chain growth in cross-linking process in the paper.