Molecular weight distribution of copolyacrylonitrile, which was obtained from precipitation copolymerization without and with using dispersants in mixed solution, is studied. The contribution ratio of liquid phase polymerization and solid phase polymerization under different polymerization conditions could be worked out through the formula, which has been deduced in literature. From the calculated results, common points of each reaction system are, i) contribution ratio (r) of solid phase to liquid phase decreases with the increase of water content; thus the solid phase polymerization is gradually strengthened, which is apt to form chain of high molecular weight, ii) the higher temperature leads to higher compatibility between water and DMSO; thus the solid phase polymerization contribution would decrease, while the value of r is considerably larger. The limit molecular weight distribution of the system without dispersants in 100% water is approaching to 2; thus the corresponding r becomes larger, the molecular weight distribution ratio (Q) decreases in the system with dispersants.