Under the pH = 2.85 phthalic acid potassium hydrogen potassium - hydrochloric acid buffer system, with vitamin C as reducing agent, copper phosphorus molybdenum yellow was reduced to copper phosphorus molybdenum blue with a maximum absorption wavelength of 700 nm. The results show that the concentration of vitamin C( x ) in the range of 0.008 ~ 0.040 mg/mL has a good linear relationship with the absorbance of the solution( y ) at 700 nm, which linear regression equation is y =17.163 x + 0.025 9, and the linear correlation coefficient r = 0.999 0. The results of the method for determinationing vitamin C in vitamin C tablets and vitamin EC granules are close to those of the Chinese Pharmacopoeia method.