Drug-protein binding is an important process in determining the activity of a pharmaceutical agent once it has entered the body. This paper developed an affinity capillary electrophoresis method to determine the binding constant between a bioactive sulfated polysaccharide 916 (916) and a potential protein, human serum albumin. This method is based on the principle that the changing analytes have different mobility shift in the zone electrophoresis. A fixed amount of protein was injected into a capillary filled with a background electrolyte containing the polysaccharide in varying concentrations. The effective mobility data of the protein were processed according to classical linearization treatments to obtain the binding constant of 916 to the HSA complex. The binding constant Ka of 916 to the human serum albumin achieved was 2.1 脳 104.