Cationic polymer fluid loss additive(CPFL)was prepared by using the reaction of 2,3-epoxypropyltrimethyl ammonium chloride(EPTMAC)(as cationic reagent) with the amide group in the molecular structure of the sodium salt of partially hydrolyzed polyacrylonitrile fibers(HPAN-Na).The chemical reaction was determined by studying the infrared absorption peaks of the materials and the products.The results proved that the cationic groups of EPTMAC were successfully grafted onto the HPAN molecular chain.The composition of the molecular chain of the product CPFL was determined by investigation and calculation of the elemental analysis results of the grafted HPAN and the final reaction product CPFL.The drilling fluid performance was evaluated,and the result showed that when the cation content was more than0.27 mmol/g,the drilling fluid would have good resistance to fluid loss and to pollution from calcium chloride. Cationic polymer fluid loss additive (CPFL) was prepared by using the reaction of 2,3-epoxypropyltrimethyl ammonium chloride (EPTMAC) (as cationic reagent) with the amide group in the molecular structure of the sodium salt of partially hydrolyzed polyacrylonitrile fibers (HPAN- Na). The chemical reaction was determined by studying the infrared absorption peaks of the materials and the products. The results proved that the cationic groups of EPTMAC were successfully grafted onto the HPAN molecular chain. The composition of the molecular chain of the product CPFL was determined by investigation and calculation of the elemental analysis results of the grafted HPAN and the final reaction product CPFL. drilling fluid performance was evaluated, and the result showed that when the cation content was more than 0.27 mmol / g, the drilling fluid would have good resistance to fluid loss and to pollution from calcium chloride.