After changing the manufacturing process of cefotaxime sodium, a new residual solvent of 2-ethylhexoic acid was introduced into the product. To control the product quality,a new GC method was established by adding 3-cyclohexanepropionic acid as an internal standard substance to detect the assay of 2-ethylhexoic acid in cefotaxime sodium. The results showed that adopting FID detector and temperature programming, 2-ethylhexoic acid and other impurities could completely be separated, the range was 0.015 007 ~3.001 4 mg/mL and correlation coefficient arrived 0.999 7, all the validation characteristics such as accuracy, precision and robustness could meet Chinese and European pharmacopoeia acceptance requirements well,the method was suitable for control of cefotaxime sodium quality.