Abstract: A spectrophotometric method has been set up to determine cefotaxime sodium using potassium ferricyanide as the spectroscopic probe reagent. With the presence of potassium ferricyanide, the degradation product of cefotaxime sodium can reduce Fe³⁺ to Fe²⁺ at pH 3.0, which facilitate the formation of soluble Prussian Blue (KFe^Ⅲ[Fe^Ⅱ(CN)₆]). The absorbance of soluble Prussian Blue is measured at its absorption maximum of 730 nm and the amount of cefotaxime sodium can be indirectly calculated. Under optimized conditions, a good linear relationship is obtained in the range of 0.040~24 mg/L of cefotaxime sodium. The linear regression equation is A=0.05088+0.2166ρ(mg/L) with linear correlation coefficient of 0.9986. The detection limit and relative standard deviation are 0.01 mg/L and 1.36%, respectively. Tthe apparent molar absorption coefficient of indirect determination of cefotaxime sodium is 2.3×10⁵ L/(mol·cm). This method has been successfully applied to the determination of cefotaxime sodium in pharmaceutical and serum samples.