PDADMAC fixing agents with different characteristics were used to control dissolved and colloidal substances in P-RC APMP fiber suspension. Results showed that PDADMAC with higher charge density was easier to interact with anionic trash, resulting in better removal of suspended solid materials and chemical oxygen demand(COD). The PDADMAC achieved 78% removal of chemical oxygen demand. PDADMAC with higher molecular weight could easily be adsorbed onto fibers and fines, and had good removal for anionic trash. It was also found that PDADMAC with higher molecular weight was better to remove suspended solid substances, while PDADMAC with lower molecular weight was more effective for removing chemical oxygen demand(COD).