Chitosan is a natural basic amino polysaccharide substance with many functional amino and hydroxyl groups on the molecular chain. So that it can become a potential flocculant. P(CS-AD) flocculant is graft copolymerized by chitosan (CS) and acrylamide (AM), acryloyloxyethyl trimethyl ammonium chloride (DAC) with UV irradiation. The effects of key factors on the intrinsic viscosity such as monomer concentration, chitosan percentage, cationic degree, photoinitiator concentration, and illumination time are discussed. The optimal synthesis conditions are determined:monomer concentration 30% to 40%, the percentage of chitosan 10%~20%, cationic degree 30%~40%, photoinitiator concentration 0.3%~0.5%, illumination time 120 min. The maximum intrinsic viscosity is 1865 mg/L after optimization. Infrared spectroscopy, scanning electron microscopy (SEM), and differential thermal analysis (TG-DTA) are used to characterize the graft copolymer. Diatomite water samples are used to verify its flocculation performance, and flocculation test results show that P (CS-AD) flocculation performance is significantly better than the commercially available polyacrylamide (PAM).